"The sacrament of the Eucharist"

"THE CHURCH'S sacraments are ordained for helping man in the spiritual life. But the spiritual life is analogous to the corporeal, since corporeal things bear a resemblance to spiritual. Now it is clear that just as generation is required for corporeal life, since thereby man receives life; and growth, whereby man is brought to maturity: so likewise food is required for the preservation of life. Consequently, just as for the spiritual life there had to be Baptism, which is spiritual generation; and Confirmation, which is spiritual growth: so there needed to be the sacrament of the Eucharist, which is spiritual food."

~St. Thomas Aquinas: Summa Theologiae, III, Q. 73, A. 1.

The Institution of the Eucharist, by Joos van Wassenhove. 

Oil on wood, 1473-75; Galleria Nazionale delle Marche, Urbino.