Jesus is God's Son. And there was never a time when He was not God's Son. Equally, there was never a time when the Father was not Father of His eternal Son, Jesus. Wind back the clock into the depths of eternity and no matter how far back you go you will always find this: The Father possessing His Son in the Spirit, The Father pouring His life into the Son by the Spirit. The Son receiving His anointing from the Father. The Son determined in the Spirit by the Father. The Father and Son have existed in a Begetting-Begotten relationship eternally. Such relationship is not simply what our God does, it's who He is. He is this eternal fellowship of the Three.
When was Christ begotten? The early church rightly answered He is 'Eternally begotten of the Father. God from God. Light from Light. True God from True God. Begotten not made. Of one being with the Father.'
Well then Psalm 2 throws up an interesting issue. Always and everywhere in Scripture Psalm 2 is said to refer to Jesus. And no matter how you get there, I hope you'll agree that it does. Well verse 7 is the Son speaking and He says this:
I will proclaim the decree of the LORD: He said to me, "You are my Son; today I have begotten you.
Well now, how do we cope with the Son of God saying such a thing? What is the 'today' on which the Son is said to be begotten? Doesn't this just collapse into Arianism? Perhaps we think the Father should have said 'Today I declare what has always been true of You - You are My Son, eternally I beget You'? But he doesn't say that. He says there's a day of begetting.
Well what day is that?
Answer: Easter Sunday. Paul correctly identifies the 'today' for us. In Acts 13:32-33 he tells us that David's intention here is to prophesy Christ's resurrection:
We tell you the good news: What God promised our fathers he has fulfilled for us, their children, by raising up Jesus. As it is written in the second Psalm: `You are my Son; today I have become your Father.'
The resurrection of Jesus is the 'today' in which the Father begets the Son. The Father and Son exist in a Begetting-Begotten relationship. And Easter is the Day on which that relationship is (and here I'm reaching for words) manifest? - too weak. Concretized? - closer. Established? - too far?
Well if we think that's too far, perhaps we also think Peter went too far in Acts 2:36. Again speaking of the resurrection he says:
God has made this Jesus, whom you crucified, both Lord and Christ.
Jesus is made Lord and Christ through the resurrection. He already was Lord (v34) and Christ (v31), yet the resurrection 'made' Him Lord and Christ.
One other Scripture to consider. In Hebrews 5, the writer sees the resurrection of Psalm 2:7 as Christ's calling to the Priesthood.
No-one takes this honour upon himself; he must be called by God, just as Aaron was. So Christ also did not take upon himself the glory of becoming a high priest. But God said to him, "You are my Son; today I have become your Father." (v4-5)
God calls Jesus to the Priesthood by raising / exalting Him. And yet at the same time Hebrews had introduced us to the eternal Son in already priestly terms (Heb 1:2,3). The Son's mediation in creation, revelation and providence is already priestly, and yet He is called to this priesthood on the basis of His death, resurrection and ascension.
This co-ordination of eternal priestliness and His historical calling continues in chapter 5. Verse 6 reminds us from Psalm 110 that Jesus is a 'priest forever in the order of [beginningless] Melchizedek'. Yet almost straight away we are told He is 'designated' priest on the basis of His suffering perfection and exaltation. (v10).
So which is it? Is Jesus eternally begotten or begotten on Easter morning? Is Jesus eternally Lord and Christ or made so by resurrection? Is Jesus eternally God's Priest or called Priest on the basis of His suffering perfection and exaltation? The answer is yes.
God's being can thus be described as a kind of being-towards-resurrection; the resurrection of Jesus is the goal of God's eternal self-determining action. In this historical (or better, this history-creating) event, God becomes what God eternally is - and this is just because God eternally is what he becomes in this event.
UPDATE: By the way, this is by no means an endorsement of Hegel. God's being is not constituted by any God-world dialectic. Rather it's the Father-Son relationship in the Spirit that constitutes God's being.