On the one hand, Jesus Christ was the only begotten Son of God from eternity to eternity (John 1:18, 3:16). John 3:16 clearly shows that Jesus is the only begotten Son of God prior to His crucifixion and resurrection. Yet on the other hand, based on Acts 13:33, Paul interprets that the resurrection of Jesus Christ is the fulfillment of the birth in Psalms 2:7. If He was already the only begotten Son of God from eternity, why would He need to be begotten again? How do we reconcile this?
We must compare Acts 13:33 with 1 Peter 1:3 and Romans 8:29:
[1Pe 1:3 NKJV]
3 Blessed [be] the God and Father of our Lord Jesus Christ, who according to His abundant mercy has begotten us again to a living hope through the resurrection of Jesus Christ from the dead,
[Rom 8:29 NKJV]
29 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.
1 Peter 1:3 shows that on the day of resurrection His believers are begotten again as well. This birth was not a small individual birth but it was a great delivery that includes humanity (all His believers) as a whole. From this perspective Paul said that He was the firstborn Son of God among many brothers (Romans 8:29).
To understand further, let's see Paul comparison of Jesus Christ with Adam in 1 Corinthians 15:45 and 47:
[1Co 15:45 NKJV]
45 And so it is written, "The first man Adam became a living being." The last Adam [became] a life-giving spirit.
[1Co 15:47 NKJV]
47 The first man [was] of the earth, [made] of dust; the second Man [is] the Lord from heaven.
On the cross, Jesus Christ brings to an end the old Adamic (all humanity) nature by being the last Adam. And through His resurrection, He was begotten as the second man, a new species of God-man, the new man in resurrection.
So what was begotten in His resurrection is His humanity. Through incarnation He brings divinity into humanity and through His resurrection, He uplifted His humanity into divinity.
Thus we can understand Romans 1:4 better:
[Rom 1:4 NKJV]
4 [and] declared [to be] the Son of God with power according to the Spirit of holiness, by the resurrection from the dead.
The word "declared" is from the Greek word horizō, which also means appointed. He was appointed to be the Son of God in power with respect to His being the Firstborn Son of God among many brothers.
This is why all those who believe into Him have the authority to become the children of God (John 1:12-13). Through the preaching of the gospel, we are "scattering" His genetic Word into the human heart. If this genetic Word is received, then it will cause an organic birth, a regeneration, a born again experience to take place deep in the receiver's being.
On the one hand, the great delivery took place two thousand years ago on the day of His resurrection, but on the other hand we are experiencing it in real time every time we preach the gospel to others today.