Christ’s Baptism Foreshadows Our Own
At first glance, the Baptism of the Lord might seem an odd feast. Since the Catholic Church teaches that the Sacrament of Baptism is necessary for the remission of sins, particularly Original Sin, why was Christ baptized? After all, He was born without Original Sin, and He lived His entire life without sinning. Therefore, He had no need of the sacrament, as we do.
In submitting Himself humbly to the baptism of St. John the Baptist, however, Christ provided the example for the rest of us. If even He should be baptized, though He had no need of it, how much more should the rest of us be thankful for this sacrament, which frees us from the darkness of sin and incorporates us into the Church, the life of Christ on earth! His Baptism, therefore, was necessary–not for Him, but for us.
Many of the Fathers of the Church, as well as the medieval Scholastics, saw Christ’s Baptism as the institution of the sacrament. His Flesh blessed the water, and the descent of the Holy Spirit (in the form of a dove) and the voice of God the Father announcing that this was His Son, in Whom He was well pleased, marked the beginning of Christ’s public ministry.
Taken from: Catholic.about.com