Transubstantiation Was Clearly Substantiated Very Early in Christianity.

He said: This is my Body; therefore the Eucharist is not the figure of his Body and Blood, as some have said, talking nonsense in their stupid minds, but it is in very truth the Blood and Body of Christ.'

  St. Macarius the Great (4th Century)

Sounds like St. Macarius probably had a rough day in the market arguing with the heretics when this was recorded. The take home message here is not the putdown of the heretic but to note the date in which he lived and said this.  Many modern protestant apologists claim that the doctrine of transubstantiation was a medieval invention of St. Thomas Aquinas (13th century), or later yet, invented at the 17th century at the Council of Trent. Obviously there is no doubt that the doctrine of  transubstantiation was clearly substantiated very early in Christianity. St. Ignatius from the early 2nd century also illustrates this profoundly important Christian belief:

"Consider how contrary to the mind of God are the heterodox in regard to the grace of God which has come to us. They have no regard for charity, none for the widow, the orphan, the oppressed, none for the man in prison, the hungry or the thirsty. They abstain from the Eucharist and from prayer, because they do not admit that the Eucharist is the flesh of our Savior Jesus Christ, the flesh which suffered for our sins and which the Father, in His graciousness, raised from the dead."