"The Sacrament of the altar"

"CHRIST'S true Body, born from the Virgin Mary, is contained in the Sacrament of the altar. To profess to the contrary is heresy, because it detracts from the truth of Scripture, which records our Lord's own words, 'This is My body' (Mt 26, 26)."

─Commentary on the Sentences: 4, 10, 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's words with faith; for since He is the Truth, He lieth not.""

─Summa Theologiae: III, q. 75, a.1. 

~St. Thomas Aquinas

The Seven Sacraments II: Eucharist, by Nicolas Poussin.
Oil on canvas, 1647; National Gallery of Scotland, Edinburgh.