Eastern Orthodox/communion prayer
What does it mean in the communion prayer when we say "I will not reveal Your mystery to your adversary"?
In the 2nd century, pagan Roman authorities sent spies into Christian congregations to observe our worship and testify in court. Among the testimony offered in court were accusations that Christians participated in ritual cannibalism, eating the Body of Christ and drinking His Blood. On this accusation, seldom denied, many were tortured and killed.
The Church responded by taking several measures.
1) Instant Baptism on demand ceased, and a three year probationary period was imposed on those seeking Baptism.
2) Catechumens seeking Baptism were assigned a sponsor to watch over them, evaluate their commitment, and teach them the basics of the Faith. This was the origin of Godparents.
3) Catechumens were allowed to witness the Liturgy of the Word, but forbidden to witness the Liturgy of the Sacrament. We still have the dismissal of the catechumens in the Liturgy.
The prayer you cite has its origin in the congregation's promise to not reveal the Mystery of Communion to pagans who would use it to persecute Christians.