Baptism of Christ

"AS Augustine says (Super Joan., Tract. xiii): "After being baptized, the Lord baptized, not with that baptism wherewith He was baptized." Wherefore, since He Himself baptized with His own baptism, it follows that He was not baptized with His own, but with John's baptism. And this was befitting: first, because John's baptism was peculiar in this, that he baptized, not in the Spirit, but only "in water"; while Christ did not need spiritual baptism, since He was filled with the grace of the Holy Ghost from the beginning of His conception, as we have made clear above (q. 34, a. 1). And this is the reason given by Chrysostom (Hom. de Bapt. Christi). Secondly, as Bede says on Mark 1:9, He was baptized with the baptism of John, that, "by being thus baptized, He might show His approval of John's baptism." Thirdly, as Gregory Nazianzen says (Orat. xxxix), "by going to John to be baptized by him, He sanctified baptism.""

~St. Thomas Aquinas: S.T.  III, q. 39, a. 2.

Baptism of Christ, by Bartolomé Esteban Murillo.
Oil on canvas,  c. 1655; Staatliche Museen, Berlin.