Today the Church celebrates a great priest and theologian who became both a saint and a Doctor of the Church. Personally, I appreciate the way in which Saint Thomas explains the Eucharist to us. No he didn't invent transubstantiation, but unpacked the richness and depth of the Eucharist so the Church could better understand, appreciate and teach what the sacrifice of the altar is all about.
Since the very beginning of the Church, the communion meal was partaking in the true body and blood of Christ, his sacrifice on the cross, re-presented in an unbloody fashion on the altar for our sins. Yes, Christ died only once on the cross, yet his sacrifice is perpetually applied to the those who available themselves of it via the partaking of the Eucharist. Here's Saint Paul: (57 AD)
"Therefore, my beloved, flee from idolatry. I speak as to wise men; you judge what I say.Is not the cup of blessing which we bless a sharing in the blood of Christ? Is not the bread which we break a sharing in the body of Christ?
Since there is one bread, we who are many are one body; for we all partake of the one bread. Look at the nation Israel; are not those who eat the sacrifices sharers in the altar? What do I mean then? That a thing sacrificed to idols is anything, or that an idol is anything? No, but I say that the things which the Gentiles sacrifice, they sacrifice to demons and not to God; and I do not want you to become sharers in demons. You cannot drink the cup of the Lord and the cup of demons; you cannot partake of the table of the Lord and the table of demons."
"For I received from the Lord that which I also delivered to you, that the Lord Jesus in the night in which He was betrayed took bread; and when He had given thanks, He broke it and said, "This is My body,
which is for you; do this in remembrance of Me." In the same way He took the cup also after supper, saying, "This cup is the new covenant in My blood; do this, as often as you drink it, in remembrance of Me."
Here's Ignatius of Antioch: (111 AD)
"Take note of those who hold heterodox opinions on the grace of Jesus Christ which has come to us, and see how contrary their opinions are to the mind of God ... They abstain from the Eucharist and from prayer because they do not confess that the Eucharist is the flesh of our Savior Jesus Christ,
flesh which suffered for our sins and which that Father, in his goodness, raised up again. They who deny the gift of God are perishing in their disputes." (Letter to the Smyrnaeans)
Saint Thomas Aquinas (13th century):
Word made flesh, the bread of nature
By his word to flesh he turns;
Wine into his blood he changes:—
What though sense no change discerns?
Only be the heart in earnest,
Faith her lesson quickly learns.