This post is a Patristic catena to provide the basis for CCC 1376:
The Council of Trent summarizes the Catholic faith by declaring: “Because Christ our Redeemer said that it was truly His Body that He was offering under the species of bread, it has always been the conviction of the Church of God, and this holy Council now declares again, that by the consecration of the bread and wine there takes place a change of the whole substance of the bread into the substance of the Body of Christ our Lord and of the whole substance of the wine into the substance of His Blood. This change the holy Catholic Church has fittingly and properly called transubstantiation.”204
204. Council of Trent (1551): DS 1642; cf. Mt 26:26 ff.; Mk 14:22 ff.; Lk 22:19 ff.; 1 Cor 11:24 ff.
For not as common bread and common drink do we receive these; but in like manner as Jesus Christ our Savior, 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.
West: Bishop St. Irenaeus of Lyons says [Against Heresies 5:2:2], “He has 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 the creation) He has established as His own Body, from which He gives increase to our bodies.”
East: Bishop St. Cyril of Jerusalem (Doctor) says in 350 [Catechetical Lectures 22:2], “He once in Cana of Galilee, turned the water into wine, akin to blood, and is it incredible that He should have turned wine into Blood?”
East: Patriarch St. Athanasius I the Great of Alexandria (Doctor) says before 373 [Sermon to the Newly Baptized in PG 26:1325C],
You will see the Levites bringing the loaves and a cup of wine, and placing them on the table. So long as the prayers and invocations have not yet been made, it is mere bread and a mere cup. But when the great and wondrous prayers have been recited, then the bread becomes the Body and the cup the Blood of our Lord Jesus Christ… When the great prayers and holy supplications are sent up, the Word descends on the bread and the cup, and it becomes His Body.
West: Bishop St. Ambrose the Great of Milan (Doctor) says in 380 [On the Christian Faith 4, 10:125],
Then He added: “For My Flesh is meat indeed, and My Blood is drink [indeed].” Thou hearest Him speak of His Flesh and of His Blood, thou perceivest the sacred pledges, [conveying to us the merits and power] of the Lord’s death, and thou dishonourest His Godhead. Hear His own words: “A spirit hath not flesh and bones.” Now we, as often as we receive the Sacramental Elements, which by the mysterious efficacy of holy prayer are transformed into the Flesh and the Blood, “do show the Lord’s Death.”
East: Bishop St. Gregory of Nyssa (Cappadocian Father) says after 383 [The Great Catechism 37],
Rightly, then, do we believe that now also the bread which is consecrated by the Word of God is changed into the Body of God the Word. For that Body was once, by implication, bread, but has been consecrated by the inhabitation of the Word that tabernacled in the flesh [εν σαρκί]. Therefore, from the same cause as that by which the bread that was transformed in that Body was changed to a Divine potency, a similar result takes place now. For as in that case, too, the grace of the Word used to make holy the Body, the substance of which came of the bread, and in a manner was itself bread, so also in this case the bread, as says the Apostle, “is sanctified by the Word of God and prayer;” not that it advances by the process of eating to the stage of passing into the Body of the Word, but it is at once changed into the Body by means of the Word, as the Word itself said, “This is My Body.”
West: Pope St. Leo I the Great of Rome (Doctor) says before 461 [Sermons 91:3 in PL 54:452AB],
Dearly beloved, utter this confession with all your heart and reject the wicked lies of heretics, that your fasting and almsgiving may not be polluted by any contagion with error: for then is our offering of the sacrifice clean and oar gifts of mercy holy, when those who perform them understand that which they do. For when the Lord says, “unless ye have eaten the flesh of the Son of Man, and drunk His Blood, ye will not have life in you,” [Jn 6:53] you ought so to be partakers at the Holy Table, as to have no doubt whatever concerning the reality of Christ’s Body and Blood. For that is taken in the mouth which is believed in Faith, and it is vain for them to respond Amen who dispute that which is taken.
The Body which is born of the holy Virgin is in truth Body united with divinity, not that the Body which was received up into the heavens descends, but that the bread itself and the wine are changed into God’s Body and Blood. But if you inquire how this happens, it is enough for you to learn that it was through the Holy Spirit, just as the Lord took on Himself flesh that subsisted in Him and was born of the holy Mother of God through the Spirit. And we know nothing further save that the Word of God is true and energizes and is omnipotent, but the manner of this cannot be searched out. But one can put it well thus, that just as in nature the bread by the eating and the wine and the water by the drinking are changed into the body and blood of the eater and drinker, and do not become a different body from the former one, so the bread of the table and the wine and water are supernaturally changed by the invocation and presence of the Holy Spirit into the Body and Blood of Christ, and are not two but one and the same.