Augustine
newadvent.org/fathers/12023.htm
“Chapter 16.— Rule for Interpreting Commands and Prohibitions.
24. If the sentence is one of command, either forbidding a crime or vice, or enjoining an act of prudence or benevolence, it is not figurative. If, however, it seems to enjoin a crime or vice, or to forbid an act of prudence or benevolence, it is figurative. Unless you eat the flesh of the Son of man, says Christ, and drink His blood, you have no life in you. John 6:53 This seems to enjoin a crime or a vice;
it is therefore a figure, enjoining that we should have a share [communicandem] in the sufferings of our Lord, and that we should retain a sweet and profitable memory [in memoria] of the fact that His flesh was wounded and crucified for us."
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Augustine**
newadvent.org/fathers/1102098.htm
“For if sacraments had not some points of real resemblance to the things of which they are the sacraments, they would not be sacraments at all. In most cases, moreover,
they do in virtue of this likeness bear the names of the realities which they resemble. As, therefore, in a certain manner the sacrament of Christ’s body is Christ’s body, and the sacrament of Christ’s blood is Christ’s blood, in the same manner the sacrament of faith is faith.”
Augustine
earlychurchtexts.com/public/augustine_sermon_272_eucharist.htm
“Inside each of you, thoughts like these are rising: “Our Lord Jesus Christ, we know the source of his flesh; he took it from the virgin Mary. Like any infant, he was nursed and nourished; he grew; became a youngster; suffered persecution from his own people. To the wood he was nailed; on the wood he died; from the wood, his body was taken down and buried. On the third day (as he willed) he rose; he ascended bodily into heaven whence he will come to judge the living and the dead. There he dwells even now, seated at God’s right. So how can bread be his body? And what about the cup? How can it (or what it contains) be his blood?” My friends, these realities are called sacraments because in them one thing is seen, while another is grasped.
What is seen is a mere physical likeness; what is grasped bears spiritual fruit.”
I have more quotes and some from different early theologians, but these are the most detailed.
Not everyone believed in a literal conversion of the substance in the early centuries. That idea did clearly win over in time.