Baptism is the sacrament of Christ's death and Passion, according as a man is born anew in Christ in virtue of His Passion; but the Eucharist is the sacrament of Christ's Passion according as a man is made perfect in union with Christ Who suffered. Hence, as Baptism is called the sacrament of Faith, which is the foundation of the spiritual life, so the Eucharist is termed the sacrament of Charity, which is "the bond of perfection" (Colossians 3:14).
[St. Thomas Aquinas, Summa Theologiae, 3.73.3 ad 3.]
