The earliest "letter" outside of the New Testament is the so-called "First Epistle of Clement to the Corinthians," which is, actually, not an "epistle (letter)" but a homily. It was written c. 96 A.D:
"In the same way, my brothers, when we offer our own Eucharist to God, each one of us should keep to his own degree. His conscience must be clear, he must not infringe the rules prescribed for his ministering, and he is to bear himself with reverence....at the altar in front of the Temple." ("Early Christian Writings," translated by Maxwell Staniforth, Penguin Classics, (1968.)
Ignatius, third Bishop of Antioch after St. Peter, on his way to martyrdom at the Flavian ampitheatre for being a Christian, wrote these pieces to ancient churches in the ancient world, c. 110 A.D.:
"[The Eucharist is] the medicine of immortality, and the sovereign remedy by which we escape death and live in Jesus Christ for evermore." (Id.)
"There is no pleasure for me in any meats that perish, or in the delights of this life; I am fain for the bread of God, even the flesh of Jesus Christ, who is the seed of David; and for my drink I crave that Blood of His which is love imperishable." (Id.)
"Make certain, therefore, that you all observe one common Eucharist; for there is but one Body of our Lord Jesus Christ, and but one cup of union with His Blood, and one single altar of sacrifice." (Id.)
"The sole Eucharist you should consider valid is one that is celebrated by the bishop himself, or by some person authorized by him." (Id.)
Saint Ignatius was sacrificed to the lions as Christ sacrificed Himself for us; that is, willing to go to death for love of others. It is no exaggeration that the Eucharist and Sacrifice that Ignatius, as Bishop, prayed so many times, is what preserved him in his martyrdom. It is through the blood of Christ, that we are Redeemed, and it is the blood of Saints such as Ignatius, shed for his fellow Christians and through love of Christ, that the seeds of Faith were planted.
