Now when all the people were baptized, it came to pass, that when Jesus also was baptized, and praying, the heaven was opened,
All Commentaries on Luke 3:21 Go To Luke 3
Augustine of Hippo
AD 430
But it is most strange that He should receive the Spirit when he was thirty years old. But as without sin He came to baptism, so not without the Holy Spirit. For if it was written of John, He shall be filled with the Spirit from his mother's womb, what must we believe of the man Christ, the very conception of whose flesh was not carnal but spiritual. Therefore He condescended now to prefigure His body, i.e. the Church, in which the baptized especially receive the Holy Spirit.
But the words of Matthew, This is my beloved Son, and those of Luke, You are my beloved, Son, convey the same meaning; for the heavenly voice spoke one of these. But Matthew wished to show that by the words, This is my beloved Son, it was meant rather to declare to the hearers, that He was the Son of God. For that was not revealed to Christ which He knew, but they heard it who were present, and for whom the voice came.
But the words of Matthew, This is my beloved Son, and those of Luke, You are my beloved, Son, convey the same meaning; for the heavenly voice spoke one of these. But Matthew wished to show that by the words, This is my beloved Son, it was meant rather to declare to the hearers, that He was the Son of God. For that was not revealed to Christ which He knew, but they heard it who were present, and for whom the voice came.