St. Thomas Aquinas
“The effect of this sacrament ought to be considered, first of all and principally, from what is contained in this sacrament, which is Christ; Who, just as by coming into the world, He visibly bestowed the life of grace upon the world, according to Jn. 1:17: ‘Grace and truth came by Jesus Christ,’ so also, by coming sacramentally into man causes the life of grace, according to Jn. 6:58: ‘He that eateth Me, the same also shall live by Me.'”
Source: St. Thomas Aquinas, Summa Theologica