St. Thomas Aquinas
“Our Lord says (Jn. 6:52): ‘The bread which I will give, is My flesh for the life of the world.’ But the spiritual life is the effect of grace. Therefore grace is bestowed through this sacrament… It is considered on the part of what is represented by this sacrament, which is Christ’s Passion, as stated above. And therefore this sacrament works in man the effect which Christ’s Passion wrought in the world.”
Source: St. Thomas Aquinas, Summa Theologica