Catholics/How can we test this for validity 2000 years later?
How do we know that Jesus's words in the new testament are ACTUALLY his words and not the words that the gospel writer and the early church stuck in his mouth as time passed? How do we test for validity 2000 years later?
The short answer to your question is that we actually have the meaning and the importance of the teaching of Jesus in the New Testament as attested by the divine inspiration of the text. Jesus entrusted His apostles/disciples to teach what He commanded to be taught as the truthful message. The validity of the message has been tested and transmitted since Jesus taught.
Often, a question also needs a somewhat lengthy answer.
Much has been written about evaluating the words of Jesus as recorded in the gospels. Here are a few basic steps toward understanding what God has revealed in the written word about what our Savior said and taught.
Timeframe: Jesus died and rose about 30 AD, when few could read. His followers [apostles, disciples] preached from their memories what He did and said. Person to person by word and memory was the usual means of communication. Memories became honed through repetition [oral tradition. Many followers in various places began to write bits and pieces about what they remembered about Jesus [written tradition]. As the generation of the first followers was beginning to close, the first extended writings about Jesus [gospels] began to appear [circa 70 AD]. While precise words are important, God was making sure [inspiration] that the precise meaning of the gospel message was being communicated in an unerring manner. God is all-knowing and all-truthful. He does not deceive. Since He had a message for all ensuing time, He would provide under His provident love that the message would not be corrupted in such an important matter.
Historical background: Jesus spoke Aramaic, a common Semitic eastern language similar to Hebrew. Most of the common people of the Mediterranean world spoke Greek, even in Rome. So, the gospels were written in Greek by the end of the first century AD.
So, already these Greek originals could hardly have been the exact words that Jesus spoke in a different language.
Gospel writers: The four authors drew upon previous oral and written traditions and their understanding of the theological message of the Messiah – additional factual reasons why we do not necessarily have the exact word of Jesus. Even in important matters, the gospels do not agree precisely. For example, the Lord’s Prayer [Our Father] has a few minor variations in the versions of Matthew [6:9 ff] and Luke [11:2ff]. Could Jesus have changed His words in various times and places? Possibly. A second example, the words of Jesus at the Last Supper over the chalice of wine [Mt 26:27-28, Mk 14:24, Lk 22:20; also, 1 Cor 11:25]. Yet, in many cases, we do have the same Greek phrasing. For example, frequently in the account of the feeding of five thousand [Mt 14:13ff, Mk 6:32ff, Lk 9:10ff, Jn 6:1ff].
Conclusion: We do not need to have the exact words of Jesus to be His followers. What we need to do is to study the inspired words of sacred scripture. We do not need to second-guess the Holy Spirit of God. We need to study the New Testament, especially the gospels, to understand and to follow the teachings of Jesus. To be more precise, in many passages we need to study carefully the original Greek – a difficult but rewarding task. Some translations more accurately present the original than others do. [Recall the axiom of Shakespearean scholars: To understand Shakespeare, you must understand him in English.]
Should you desire more, check Raymond Collin, Introduction to the New Testament [Garden City: Doubleday, 1983], available at least through inter-library loan.
If you wish more from me, please feel free to write again.
May God bless you, Charles.