14 Jesus Christ was the Son of God, yet He preached not the Gospel before His Baptism. If the Master Himself followed the right time in due order, ought we, His servants, to venture out of order? From that time Jesus began to preach , when the Holy Spirit had descended upon Him in a bodily shape, like a dove; not that Jesus might see Him first, for He knew Him even before He came in a bodily shape, but that John, who was baptizing Him, might behold Him.
For I, says he, knew Him not: but He that sent me to baptize with water, He said unto me, Upon whomsoever you shall see the Spirit descending and abiding on Him, that is He. If you too hast unfeigned piety, the Holy Ghost comes down on you also, and a Father's voice sounds over you from on high— not, “This is My Son,” but, “This has now been made My son;” for the “is” belongs to Him alone, because In the beginning was the Word, and the Word was with God, and the Word was God. To Him belongs the “is,” since He is always the Son of God: but to you “has now been made:” since you have not the sonship by nature, but receive it by adoption. He eternally “is;” but you receive the grace by advancement.
Source: Catechetical Lectures (New Advent)