The word transubstantiation was not written out in detail until the 13th century. But the early Church Fathers believed in it in its entirety. Here is only but a few:
“They abstain from the Eucharist and from prayer, because they confess not the Eucharist to be the flesh of our Saviour Jesus Christ, which suffered for our sins, and which the Father, of His goodness, raised up again.” Ignatius of Antioch, Epistle to Smyrnaeans, 7,1 (c. A.D. 110).
“For not as common bread and common drink do we receive these; but in like manner as Jesus Christ our Saviour, having been made flesh and blood for our salvation, so likewise have we been taught that the food which is blessed by the prayer of His word, and from which our blood and flesh by transmutation are nourished, is the flesh and blood of that Jesus who was made flesh.” Justin Martyr, First Apology, 66 (c. A.D. 110-165).
“[T]he bread over which thanks have been given is the body of their Lord, and the cup His blood…” Irenaeus, Against Heresies, IV:18,4 (c. A.D. 200).
“He acknowledged the cup (which is a part of the creation) as his own blood, from which he bedews our blood; and the bread (also a part of creation) he affirmed to be his own body, from which he gives increase to our bodies.” Irenaeus, Against Heresies, V:2,2 (c. A.D. 200).
“But what consistency is there in those who hold that the bread over which thanks have been given is the Body of their Lord, and the cup His Blood, if they do not acknowledge that He is the Son of the Creator of the world…” Irenaeus, Against Heresies, IV:18, 2 (c. A.D. 200).
From wikipedia: “In 1215, the Fourth Lateran Council used the word transubstantiated in its profession of faith, when speaking of the change that takes place in the Eucharist.”
Also if the Catholic Church didn’t believe in the Real Presence in the Eucharist until 1215, then why do ALL of the Orthodox Churches believe in it, when the Churches split in 1054?