Jesus said to them, “Amen, amen, I say to you, unless you eat the flesh of the Son of Man and drink his blood, you do not have life within you. Whoever eats my flesh and drinks my blood has eternal life, and I will raise him on the last day. For my flesh is TRUE FOOD, and my blood is TRUE DRINK. Whoever eats my flesh and drinks my blood remains in me and I in him.  —  John 6:53-56

By the consecration of the bread and of the wine a conversion is made 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; which conversion is by the holy Catholic Church suitably and properly called TRANSUBSTANTIATION.   —-   Council of Trent, Decree Concerning the Most Holy Sacrament of the Eucharist, Ch. 4, On Transubstantiation, October 11, 1551

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.” (Council of Trent, 1551; cf. Mt 26:26ff; Mk 14:22ff; Lk 22:19ff; 1 Cor 11:24ff)  —-   Catechism of the Catholic Church, 1376

People say that the doctrine of Transubstantiation is difficult to believe… It is difficult, impossible to imagine, I grant – but how is it difficult to believe?… For myself, I cannot indeed prove it, I cannot tell how it is; but I say, “Why should it not be? What’s to hinder it? What do I know of substance or matter? Just as much as the greatest philosophers, and that is nothing at all.”…And, in like manner… the doctrine of the Trinity in Unity. What do I know of the Essence of the Divine Being? I know that my abstract idea of three is simply incompatible with my idea of one; but when I come to the question of concrete fact, I have no means of proving that there is not a sense in which one and three can equally be predicated of the Incommunicable God.  —   John Henry Cardinal Newman, Apologia Pro Vita Sua, 318, Double Day Image

At the Last Supper, before his Crucifixion, Jesus offered the disciples bread and wine. Jesus did not say, “This bread contains my body” or “I am in this wine.” Jesus simply said of the bread, “Take, eat; this is my body” and of the wine, “Drink of it, all of you; for this is my blood of the covenant” (Matthew 26:26-28). The Church teaches that when they are consecrated at Mass, the substance of these things changes into the body and blood of Christ, although the appearances of the bread and wine remain.
-Trent Horn, Why We’re Catholic: Our Reasons for Faith. Hope and Love