How would you explain the trinity.
Is Jesus the Almighty God
The first and best exposition of the doctrine of the Holy Trinity was none other than that of His Holiness Pope Leo I ("the Great"), a document commonly referred to as "the Tome." In this he responds to his Flavian, his fellow worker in the Lord's harvest regarding the heresies of one heretical priest named Eutyches.
It may seem long and verbose and with valuable space squandered on reiterating the heresies of Eutyches, but in this is practically the bedrock for the doctrine whole and entire, the only further possible bedrock being the sources subsequently quoted by the same Pope after giving the foregoing as I enclose here in full. I cannot imagine a better and more reliable source for all my beliefs than the Church, and the Successor of Peter who is here heard through Leo defining in detail the doctrine of the Holy Trinity:
The Tome of St. Leo.
(From his Select Sermons of S. Leo the Great on the Incarnation with his XXVIIIth Epistle called the "Tome.")
Leo [the bishop] to his [most] dear brother Flavian:
Having read your Affection's letter, the late arrival of which is matter of surprise to us, and having gone through the record of the proceedings of the bishops, we have now, at last, gained a clear view of the scandal which has risen up among you, against the integrity of the faith; and what at first seemed obscure has now been elucidated and explained. By this means Eutyches, who seemed to be deserving of honor under the title of Presbyter, is now shown to be exceedingly thoughtless and sadly inexperienced, so that to him also we may apply the prophet's words, "He refused to understand in order to act well: he meditated unrighteousness on his bed." What, indeed, is more unrighteous than to entertain ungodly thoughts, and not to yield to persons wiser and more learned? But into this folly do they fall who, when hindered by some obscurity from apprehending the truth, have recourse, not to the words of the Prophets, not to the letters of the Apostles, nor to the authority of the Gospels, but to themselves; and become teachers of error, just because they have not been disciples of the truth. For what learning has he received from the sacred pages of the New and the Old Testament, who does not so much as understand the very beginning of the Creed? And that which, all the world over, is uttered by the voices of all applicants for regeneration, is still not grasped by the mind of this aged man. If, then, he knew not what he ought to think about the Incarnation of the Word of God, and was not willing, for the sake of obtaining the light of intelligence, to make laborious search through the whole extent of the Holy Scriptures, he should at least have received with heedful attention that general Confession common to all, whereby the whole body of the faithful profess that they "believe in God the Father Almighty, and in Jesus Christ his only Son our Lord, who was born of the Holy Ghost and the Virgin Mary." By which three clauses the engines of almost all heretics are shattered. For when God is believed to be both "Almighty" and "Father," it is proved that the Son is everlasting together with himself, differing in nothing from the Father, because he was born as "God from God," Almighty from Almighty, Coeternal from Eternal; not later in time, not inferior in power, not unlike him in glory, not divided from him in essence, but the same Only-begotten and Everlasting Son of an Everlasting Parent was "born of the Holy Ghost and the Virgin Mary." This birth in time in no way detracted from, in no way added to, that divine and everlasting birth; but expended itself wholly in the work of restoring man, who had been deceived; so that it might both overcome death, and by its power "destroy the devil who had the power of death." For we could not have overcome the author of sin and of death, unless he who could neither be contaminated by sin, nor detained by death, had taken upon himself our nature, and made it his own. For, in fact, he was "conceived of the Holy Ghost" within the womb of a Virgin Mother, who bore him as she had conceived him, without loss of virginity. It will be noticed here that the virgin-birth is as distinctly defined as the virgin-conception. But if he (Eutyches) was not able to obtain a true conception from this pure fountain of Christian faith because by his own blindness he had darkened for himself the brightness of a truth so clear, he should have submitted himself to the Evangelist's teaching; and after reading what Matthew says, "The book of the generation of Jesus Christ, the Son of David, the Son of Abraham," he should also have sought instruction from the Apostle's preaching; and after reading in the Epistle to the Romans, "Paul, a servant of Jesus Christ, called an Apostle, separated unto the gospel of God, which he had promised before by the prophets in the Holy Scriptures, concerning his Son, who was made unto him of the seed of David according to the flesh," he should have bestowed some devout study on the pages of the Prophets; and finding that God's promise said to Abraham, "in thy seed shall all nations be blessed," in order to avoid all doubt as to the proper meaning of this "seed," he should have attended to the Apostle's words, "To Abraham and to his seed were the promises made. He says not, 'and to seeds,' as in the case of many, but as in the case of one, 'and to thy seed,' which is Christ." He should also have apprehended with his inward ear the declaration of Isaiah, "Behold, a Virgin shall conceive and bear a Son, and they shall call his name Emmanuel, which is, being interpreted, God with us;" and should have read with faith the words of the same prophet, "Unto us a Child has been born, unto us a Son has been given, whose power is on his shoulder; and they shall call his name Angel of great counsel, Wonderful, Counselor, Strong God, Prince of Peace, Father of the age to come." And he should not have spoken idly to the effect that the Word was in such a sense made flesh, that the Christ who was brought forth from the Virgin's womb had the form of a man, and had not a body really derived from his Mother's body. Possibly his reason for thinking that our Lord Jesus Christ was not of our nature was this—that the Angel who was sent to the blessed and ever Virgin Mary said, "The Holy Ghost shall come upon thee, and the power of the Highest shall overshadow thee, and therefore also that holy thing which shall be born of thee shall be called the Son of God;" as if, because the Virgin's conception was caused by a divine act, therefore the flesh of him whom she conceived was not of the nature of her who conceived him. But we are not to understand that "generation," peerlessly wonderful, and wonderfully peerless, in such a sense as that the newness of the mode of production did away with the proper character of the kind. For it was the Holy Ghost who gave fecundity to the Virgin, but it was from a body that a real body was derived; and "when Wisdom was building herself a house," the "Word was made flesh, and dwelt among us," that is, in that flesh which he assumed from a human being, and which he animated with the spirit of rational life.
Accordingly while the distinctness of both natures and substances was preserved, and both met in one Person, lowliness was assumed by majesty, weakness by power, mortality by eternity; and, in order to pay the debt of our condition, the inviolable nature was united to the passible, so that as the appropriate remedy for our ills, one and the same "Mediator between God and man, the Man Christ Jesus," might from one element be capable of dying and also from the other be incapable. Therefore in the entire and perfect nature of very man was born very God, whole in what was his, whole in what was ours. By "ours" we mean what the Creator formed in us at the beginning and what he assumed in order to restore; for of that which the deceiver brought in, and man, thus deceived, admitted, there was not a trace in the Savior; and the fact that he took on himself a share in our infirmities did not make him a partaker in our transgressions. He assumed "the form of a servant" without the defilement of sin, enriching what was human, not impairing what was divine: because that "emptying of himself," whereby the Invisible made himself visible, and the Creator and Lord of all things willed to be one among mortals, was a stooping down in compassion, not a failure of power. Accordingly, the same who, remaining in the form of God, made man, was made man in the form of a servant. For each of the natures retains its proper character without defect; and as the form of God does not take away the form of a servant, so the form of a servant does not impair the form of God. For since the devil was glorying in the fact that man, deceived by his craft, was bereft of divine gifts and, being stripped of his endowment of immortality, had come under the grievous sentence of death, and that he himself, amid his miseries, had found a sort of consolation in having a transgressor as his companion, and that God, according to the requirements of the principle of justice, had changed his own resolution in regard to man, whom he had created in so high a position of honor; there was need of a dispensation of secret counsel, in order that the unchangeable God, whose will could not be deprived of its own benignity, should fulfill by a more secret mystery his original plan of loving kindness toward us, and that man, who had been led into fault by the wicked subtlety of the devil, should not perish contrary to God's purpose. Accordingly, the Son of God, descending from his seat in heaven, and not departing from the glory of the Father, enters this lower world, born after a new order, by a new mode of birth. After a new order; because he who in his own sphere is invisible, became visible in ours; He who could not be enclosed in space, willed to be enclosed; continuing to be before times, he began to exist in time; the Lord of the universe allowed his infinite majesty to be overshadowed, and took upon him the form of a servant; the impassible God did not disdain to be passible Man and the immortal One to be subjected to the laws of death. And born by a new mode of birth; because inviolate virginity, while ignorant of concupiscence, supplied the matter of his flesh. What was assumed from the Lord's mother was nature, not fault; nor does the wondrousness of the nativity of our Lord Jesus Christ, as born of a Virgin's womb, imply that his nature is unlike ours. For the selfsame who is very God, is also very man; and there is no illusion in this union, while the lowliness of man and the loftiness of Godhead meet together. For as "God" is not changed by the compassion [exhibited], so "Man" is not consumed by the dignity [bestowed]. For each "form" does the acts which belong to it, in communion with the other; the Word, that is, performing what belongs to the Word, and the flesh carrying out what belongs to the flesh; the one of these shines out in miracles, the other succumbs to injuries. And as the Word does not withdraw from equality with the Father in glory, so the flesh does not abandon the nature of our kind. For, as we must often be saying, he is one and the same, truly Son of God, and truly Son of Man. God, inasmuch as "in the beginning was the Word, and the Word was with God, and the Word was God." Man, inasmuch as "the Word was made flesh, and dwelt among us." God, inasmuch as "all things were made by him, and without him nothing was made." Man, inasmuch as he was "made of a woman, made under the law." The nativity of the flesh is a manifestation of human nature; the Virgin's child-bearing is an indication of Divine power. The infancy of the Babe is exhibited by the humiliation of swaddling clothes: the greatness of the Highest is declared by the voices of angels. He whom Herod impiously designs to slay is like humanity in its beginnings; but he whom the Magi rejoice to adore on their knees is Lord of all. Now when he came to the baptism of John his forerunner, lest the fact that the Godhead was covered with a veil of flesh should be concealed, the voice of the Father spoke in thunder from heaven, "This is my beloved Son, in whom I am well pleased." Accordingly, he who, as man, is tempted by the devil's subtlety, is the same to whom, as God, angels pay duteous service. To hunger, to thirst, to be weary, and to sleep, is evidently human. But to satisfy five thousand men with five loaves, and give to the Samaritan woman that living water, to draw which can secure him that drinks of it from ever thirsting again; to walk on the surface of the sea with feet that sink not, and by rebuking the storm to bring down the "uplifted waves," is unquestionably Divine. As then–to pass by many points–it does not belong to the same nature to weep with feelings of pity over a dead friend and, after the mass of stone had been removed from the grave where he had lain four days, by a voice of command to raise him up to life again; or to hang on the wood, and to make all the elements tremble after daylight had been turned into night; or to be transfixed with nails, and to open the gates of paradise to the faith of the robber; so it does not belong to the same nature to say, "I and the Father are one," and to say, "the Father is greater than I." For although in the Lord Jesus Christ there is one Person of God and man, yet that whereby contumely attaches to both is one thing, and that whereby glory attaches to both is another; for from what belongs to us he has that manhood which is inferior to the Father; while from the Father he has equal Godhead with the Father. Accordingly, on account of this unity of Person which is to be understood as existing in both the natures, we read, on the one hand, that "the Son of Man came down from heaven," inasmuch as the Son of God took flesh from that Virgin of whom he was born; and on the other hand, the Son of God is said to have been crucified and buried, inasmuch as he underwent this, not in his actual Godhead; wherein the Only-begotten is coeternal and consubstantial with the Father, but in the weakness of human nature. Wherefore we all, in the very Creed, confess that "the only-begotten Son of God was crucified and buried," according to that saying of the Apostle, "for if they had known it, they would not have crucified the Lord of Majesty."
But when our Lord and Savior himself was by his questions instructing the faith of the disciples, he said, "Whom do men say that I the Son of Man am?" And when they had mentioned various opinions held by others, he said, "But whom say ye that I am?" that is, "I who am Son of Man, and whom you see in the form of a servant, and in reality of flesh, whom say ye that I am?" Whereupon the blessed Peter, as inspired by God, and about to benefit all nations by his confession, said, "Thou art the Christ, the Son of the living God." Not undeservedly, therefore, was he pronounced blessed by the Lord, and derived from the original Rock that solidity which belonged both to his virtue and to his name, who through revelation from the Father confessed the selfsame to be both the Son of God and the Christ; because one of these truths, accepted without the other, would not profit unto salvation, and it was equally dangerous to believe the Lord Jesus Christ to be merely God and not man, or merely man and not God. But after the resurrection of the Lord–which was in truth the resurrection of a real body, for no other person was raised again than he who had been crucified and had died–what else was accomplished during that interval of forty days than to make our faith entire and clear of all darkness? For while he conversed with his disciples, and dwelt with them, and ate with them, and allowed himself to be handled with careful and inquisitive touch by those who were under the influence of doubt, for this end he came in to the disciples when the doors were shut, and by his breath gave them the Holy Ghost, and opened the secrets of Holy Scripture after bestowing on them the light of intelligence, and again in his selfsame person showed to them the wound in the side, the prints of the nails, and all the flesh tokens of the Passion, saying, "Behold my hands and my feet, that it is I myself; handle me and see, for a spirit hath not flesh and bones, as ye see me have:" that the properties of the Divine and the human nature might be acknowledged to remain in him without causing a division, and that we might in such sort know that the Word is not what the flesh is, as to confess that the one Son of God is both Word and flesh. On which mystery of the faith this Eutyches must be regarded as unhappily having no hold, who does not recognize our nature to exist in the Only-begotten Son of God, either by way of the lowliness of mortality, or of the glory of resurrection. Nor has he been overawed by the declaration of the blessed Apostle and Evangelist John, saying, "Every spirit that confesses that Jesus Christ has come in the flesh is of God; and every spirit which dissolves Jesus is not of God, and this is Antichrist." Now what is to dissolve Jesus, but to separate the human nature from him, and to make void by shameless inventions that mystery by which alone we have been saved? Moreover, being in the dark as to the nature of Christ's body, he must needs be involved in the like senseless blindness with regard to his Passion also. For if he does not think the Lord's crucifixion to be unreal, and does not doubt that he really accepted suffering, even unto death, for the sake of the world's salvation; as he believes in his death, let him acknowledge his flesh also, and not doubt that he whom he recognizes as having been capable of suffering is also Man with a body like ours; since to deny his true flesh is also to deny his bodily sufferings. If then he accepts the Christian faith, and does not turn away his ear from the preaching of the Gospel, let him see what nature it was that was transfixed with nails and hung on the wood of the cross; and let him understand whence it was that, after the side of the Crucified had been pierced by the soldier's spear, blood and water flowed out, that the Church of God might be refreshed both with a Laver and with a Cup. Let him listen also to the blessed Apostle Peter when he declares, that "sanctification by the Spirit" takes place through the "sprinkling of the blood of Christ," and let him not give a mere cursory reading to the words of the same Apostle, "Knowing that ye were not redeemed with corruptible things, as silver and gold, from your vain way of life received by tradition from your fathers, but with the precious blood of Jesus Christ as of a Lamb without blemish and without spot." Let him also not resist the testimony of Blessed John the Apostle, "And the blood of Jesus the Son of God cleanses us from all sin." And again, "This is the victory which overcomes the world, even our faith;" and, "who is he that overcomes the world, but he that believeth that Jesus is the Son of God? This is he that came by water and blood, even Jesus Christ; not in water only, but in water and blood; and it is the Spirit that bears witness, because the Spirit is truth. For there are three that bear witness—the Spirit, the water, and the blood; and the three are one." That is, the Spirit of sanctification, and the blood of redemption, and the water of baptism; which three things are one, and remain undivided, and not one of them is disjoined from connection with the others; because the Catholic Church lives and advances by this faith, that Christ Jesus we should believe neither manhood to exist without true Godhead, nor Godhead without true manhood. But when Eutyches, on being questioned in your examination of him, answered, "I confess that our Lord was of two natures before the union, but after the union I confess one nature;" I am astonished that so absurd and perverse a profession as this of his was not rebuked by a censure on the part of any of his judges, and that an utterance extremely foolish and extremely blasphemous was passed over, just as if nothing had been heard which could give offence: seeing that it is as impious to say that the Only-begotten Son of God was of two natures before the Incarnation as it is shocking to affirm that, since the Word became flesh, there has been in him one nature only. But lest Eutyches should think that what he said was correct, or was tolerable, because it was not confuted by any assertion of yours, we exhort your earnest solicitude, dearly beloved brother, to see that, if by God's merciful inspiration the case is brought to a satisfactory issue, the inconsiderate and inexperienced man be cleansed also from this pestilent notion of his; seeing that, as the record of the proceedings has clearly shown, he had fairly begun to abandon his own opinion when on being driven into a corner by authoritative words of yours, he professed himself ready to say what he had not said before, and to give his adhesion to that faith from which he had previously stood aloof. But when he would not consent to anathematize the impious dogma you understood, brother, that he continued in his own misbelief, and deserved to receive sentence of condemnation. For which if he grieves sincerely and to good purpose, and understands, even though too late, how properly the Episcopal authority has been put in motion, or if, in order to make full satisfaction, he shall condemn viva voce, and under his own hand, all that he has held amiss, no compassion, to whatever extent, which can be shown him when he has been set right, will be worthy of blame, for our Lord, the true and good Shepherd, who laid down his life for his sheep, and who came to save men's souls and not to destroy them, wills us to imitate his own loving kindness; so that justice should indeed constrain those who sin, but mercy should not reject those who are converted. For then indeed is the true faith defended with the best results, when a false opinion is condemned even by those who have followed it. But in order that the whole matter may be piously and faithfully carried out, we have appointed our brethren, Julius, Bishop, and Reatus, Presbyter (of the title of St. Clement) and also my son Hilarus, Deacon, to represent us; and with them we have associated Dulcitius, our Notary, of whose fidelity we have had good proof: trusting that the Divine assistance will be with you, so that he who has gone astray may be saved by condemning his own unsound opinion. May God keep you in good health, dearly beloved brother. Given on the Ides of June, in the Consulate of the illustrious men, Asterius and Protogenes.