What this section shows is that Jesus actually became a born-again Son of God through a resurrection from the dead. Before His resurrection He was a son of David, born into that line by physical birth. His ancestry is meticulously recorded, showing His descent from Adam, the son of God (Luke 3:23-38). Thus, He was indeed the Son of God prior to the resurrection of the dead by means of human birth.
Though Romans 1:3 plainly shows His physical generation, verse 4, in contrast, shows His spiritual generation. We see Christ in two different positions, born first as a human, then born as God. Why did Paul differentiate between the two if a birth is not involved in each? He is showing that at the resurrection, our Lord and Savior became the Son of God just as we will—by a resurrection from the dead!
Paul writes more about this birth in Romans 8:29, "For whom He foreknew, He also predestined to be conformed to the image of His Son that He might be the firstborn [prototokos] among many brethren." Who are His brethren? His disciples, members of His body, the church! We will be born again! We have already been born one time, even as Jesus was born to Mary. But He was declared to be a Son, born by spiritual generation, and we will follow that pattern.
Jesus Christ was born again! He is the pattern. His brethren, converted Christians, will go through the same born-again experience that He did. Consider that He never had to repent, be baptized (although He was, to set an example for us), have hands laid on Him to receive the Spirit, or be converted, yet He was born again. But He was not born again until He was composed of spirit.
This reveals when being born again takes place! For us, it occurs precisely when it occurred for the pattern, Jesus Christ. He was not a born-again Son of God until His resurrection from the dead, and we will not be born again until our resurrection from the dead (I Corinthians 15:50-54)!
Since Jesus did not have to be converted, and since He was declared God's Son by His resurrection, being converted is not being born again. All of God's children will follow the same pattern as the Firstborn. We will be born again the same way, by the resurrection from the dead. "And if children, then heirs—heirs of God and joint heirs with Christ, if indeed we suffer with Him, that we may also be glorified together" (verse 17). When did Christ become glorified? At His resurrection! We will be glorified together. When? At our resurrection! We will follow the pattern.