Here is what the Church Fathers have said:
St. Ignatius of Antioch (c. 110 A.D.)
I have no taste for corruptible food nor for the pleasures of this life.
I desire the Bread of God, WHICH IS THE FLESH OF JESUS CHRIST, who was of the seed of David; and for drink I DESIRE HIS BLOOD, which is love incorruptible. (Letter to the Romans 7:3)
St. Justin the Martyr (c. 100 - 165 A.D.)
For not as common bread nor common drink do we receive these; but since Jesus Christ our Savior was made incarnate by the word of God and had both flesh and blood for our salvation, so too,
as we have been taught, the food which has been made into the Eucharist by the Eucharistic prayer set down by Him, AND BY THE CHANGE OF WHICH our blood and flesh is nourished, IS BOTH THE FLESH AND THE BLOOD OF THAT INCARNATED JESUS. (First Apology, 66)
St. Irenaeus of Lyons (c. 140 - 202 A.D.)
If the BODY be not saved, then, in fact, neither did the Lord redeem us with His BLOOD; and neither is the cup of the EUCHARIST THE PARTAKING OF HIS BLOOD nor is the bread which we break THE PARTAKING OF HIS BODY…
He has declared the cup, a part of creation, TO BE HIS OWN BLOOD, from which He causes our blood to flow; and the bread, a part of creation, HE HAS ESTABLISHED AS HIS OWN BODY, from which He gives increase to our bodies.
St. Clement of Alexandria (c. 150 - 216 A.D.)
“EAT MY FLESH,” He says, “AND DRINK MY BLOOD.”
The Lord supplies us with these intimate nutriments. HE DELIVERS OVER HIS FLESH, AND POURS OUT HIS BLOOD; and nothing is lacking for the growth of His children. O incredible mystery! (Instructor of Children 1:6:42,1,3)
St. Athanasius (c. 295 - 373 A.D.)
But after the great and wonderful prayers have been completed, then the bread is become the Body, and the wine the Blood, of our Lord Jesus Christ….Let us approach the celebration of the mysteries. This bread and this wine, so long as the prayers and supplications have not taken place, remain simply what they are.
But after the great prayers and holy supplications have been sent forth, the Word comes down into the bread and wine – and thus is His Body confected. (Sermon to the Newly Baptized, from Eutyches)
St. Jerome (c. 347 - 420 A.D.)
After the type had been fulfilled by the passover celebration and He had eaten the flesh of the lamb with His Apostles, He takes bread which strengthens the heart of man, and goes on to the true Sacrament of the passover, so that just as Melchisedech, the priest of the Most High God, in prefiguring Him, made bread and wine an offering,
He too makes Himself manifest in the reality of His own Body and Blood. (Commentaries on Matthew 4:26:26)
St. Augustine (c. 354 - 430 A.D.)
“The Lord Jesus wanted those whose eyes were held lest they should recognize him, to recognize Him in the breaking of the bread [Luke 24:16,30-35]. The faithful know what I am saying.
They know Christ in the breaking of the bread. For not all bread, but only that which receives the blessing of Christ, BECOMES CHRIST’S BODY.” (Sermons 234:2)
Source:
Fathers of the Church on the Eucharist