It is interesting that, in all of Paul's sermon in Acts 13, other than in this citation, where sonship is not the point Paul is seeking to prove, the sonship of Christ is not mentioned once (Jesus is merely referred to as "a Savior—Jesus"). In verse 33 (cited above), Paul does not quote Psalm 2 as a proof of Jesus' sonship (since this is not the point he is seeking to establish in the citation), but as a proof of Christ's resurrection.God has fulfilled this for us their children, in that He has raised up Jesus. As it is also written in the second Psalm:
‘You are My Son,
Today I have begotten You.’
That Jesus was "begotten" as "firstborn" from the dead is also affirmed in Col.1:18 and Rev.1:5:
And He is the head of the body, the church, who is the beginning, the firstborn from the dead, that in all things He may have the preeminence.
The concept is expressed in different imagery in 1 Corinthians 15:20:and from Jesus Christ, the faithful witness, the firstborn from the dead, and the ruler over the kings of the earth.
There were no days before the creation of the world (Genesis 1:5). "This day I have begotten you," was the day Jesus emerged from the grave as the first one of "many brethren" to be born into the New Creation (Romans 8:29):But now Christ is risen from the dead, and has become the firstfruits of those who have fallen asleep.
For whom He foreknew, He also predestined to be conformed to the image of His Son, that He might be the firstborn among many brethren.