"Secondly, that Christ might be manifested. Whence John himself says (Jn 1:31): "That He," i.e. Christ, "may be made manifest in Israel, therefore am I come baptizing with water." For he announced Christ to the crowds that gathered around him; which was thus done much more easily than if he had gone in search of each individual, as Chrysostom observes, commenting on St. John (Hom. x in Mt.).
"Thirdly, that by his baptism he might accustom men to the baptism of Christ; wherefore Gregory says in a homily (Hom. vii in Evang.) that therefore did John baptize, "that, being consistent with his office of precursor, as he had preceded our Lord in birth, so he might also by baptizing precede Him who was about to baptize."
"Fourthly, that by persuading men to do penance, he might prepare men to receive worthily the baptism of Christ. Wherefore Bede [Cf. Scot. Erig. in Joan. iii, 24] says that "the baptism of John was as profitable before the baptism of Christ, as instruction in the faith profits the catechumens not yet baptized. For just as he preached penance, and foretold the baptism of Christ, and drew men to the knowledge of the Truth that hath appeared to the world, so do the ministers of the Church, after instructing men, chide them for their sins, and lastly promise them forgiveness in the baptism of Christ." "
~St. Thomas Aquinas: Summa Theologica, III, q. 38, a. 1.
The Preaching of Saint John the Baptist, by Bacchiacca.
Oil on wood, c. 1520; Szépmûvészeti Múzeum, Budapest.
