It’s always amazing that the “fundamental” Protestants pass over this, but they do. From Thomas Aquinas, Summa Theologiae, 3 q.75 a.1:
The presence of Christ’s true body and blood in this sacrament cannot be detected by sense, nor understanding, but by faith alone, which rests upon Divine authority. Hence, on Luke 22:19: “This is My body which shall be delivered up for you,” Cyril says: “Doubt not whether this be true; but take rather the Saviour’swords with faith; for since He is the Truth, He lieth not.”
Now this is suitable, first for the perfection of the New Law. For, the sacrifices of the Old Law contained only in figure that true sacrifice of Christ’s Passion, according to Hebrews 10:1: “For the law having a shadow of the good things to come, not the very image of the things.” And therefore it was necessary that the sacrifice of the New Law instituted by Christ should have something more, namely, that it should contain Christ Himself crucified, not merely in signification or figure, but also in verytruth. And therefore this sacrament which contains Christ Himself, as Dionysius says (Eccl. Hier. iii), is perfective of all the other sacraments, in which Christ’s virtue is participated.
Secondly, this belongs to Christ’s love, out of which for our salvation He assumed a true body of our nature. And because it is the special feature of friendship to live together with friends, as the Philosopher says (Ethic. ix), He promises us His bodily presence as a reward, saying (Matthew 24:28): “Where the body is, there shall the eagles be gathered together.” Yet meanwhile in our pilgrimage He does not deprive us of His bodily presence; but unites us with Himself in this sacrament through the truth of His body and blood. Hence (John 6:57) he says: “He that eateth My flesh, and drinketh My blood, abideth in Me, and I in him.” Hence this sacrament is the sign of supreme charity, and the uplifter of our hope, from such familiar union of Christ with us.
Thirdly, it belongs to the perfection of faith, which concerns His humanity just as it does His Godhead, according to John 14:1: “You believe in God, believe also in Me.” And since faith is of things unseen, as Christ shows us His Godhead invisibly, so also in this sacrament He shows us His flesh in an invisible manner.
Although Aquinas starts with it as a matter of faith, he doesn’t let the matter rest there, proceeding with reasons why a proposition is so. In addition to a truly Biblical eucharistic theology, his method separates him from many others in that he always wants to answer the question “why,” a question that’s getting harder and harder to even ask these days.