"This day have I begotten thee"
This is one of the most striking and puzzling phrases in Scripture. The implication is one of adoption (today I become your father), but the phrase is, I believe, correctly translated in the RSV as implying an act of procreation, albeit a retrospective one.
We are perhaps most familiar with it from Heb 1:5 (quoted in Handels Messiah)
For to what angel did God ever say, "Thou art my Son,
Today I have begotten thee"?
In the context (and again in 5:5) the phrase clearly is being applied to Jesus in risen glory. But any Hebrew recipient of this letter would be able to answer: no angel, but both David and Solomon. In Acts 13:33 the verse is again quoted as "from the second psalm" and taken as a prophecy about Jesus. But this is a psalm of David:
Ps 2:7a: I will tell of the decree of the LORD:
Ps 2:6: "I have set my king
On Zion, my holy hill."
Ps 2:7b: He said to me, "You are my son,
Today I have begotten you."
And moreover, David is told a prophecy concerning Solomon:
2 Sam 7:14=1 Chron 17:13: [God says] I will be his father, and he shall be my son.
In the context, this is a royal title, as the surrounding verses speak of establishing his kingdom for ever. The identity as Solomon is implied by 2 Sam 7:13=1 Chron 17:12. (Solomon built the temple, Zerubabbel rebuilt it but was not a king, Jesus created a new temple not made with hands: see another article.)
The royal title is predicted for Jesus by Gabriel at the Annunciation (Lk 1:32-33)
He will be great, and will be called the Son of the Most High;
and the Lord God will give him the throne of his father David,
and he will reign over the house of Jacob for ever;
and of his kingdom there will be no end.
where the connection between sonship and royalty is again implicit (the alternate lines as in much Hebrew poetry acting as echoes). However, the description of his reign and kingdom is not a reassuring one: there are no more earthly kings of this line.
Intriguingly, the voice from heaven that appears at Jesus baptism (Mk 1:11=Mt 3:17=Lk 3:22) and elsewhere in the gospels (and 2 Pet 1:17) is rendered in the Greek as "This is my son, the beloved, in whom I am well pleased." But the Aramaic version (on every occurrence) is closer to the Psalm: "This is my son by whom I am fulfilled!" According to the Revised Standard Version, in some manuscripts Lk 3:22 continues "today I have begotten thee".
Malcolm Crowe, Glasgow, 11 June - 12 October 1998.