“And declared to be the Son of God with power, according to the spirit of holiness, by the resurrection from the dead:”
King James Version (KJV)
1:4 But powerfully declared to be the Son of God, according to the Spirit of Holiness - That is, according to his divine nature. By the resurrection from the dead - For this is both the fountain and the object of our faith; and the preaching of the apostles was the consequence of Christ's resurrection.
Ro 1:4 But declared [to be] the Son of God. Though in human form he was demonstrated to be divine by power, such power as he displayed in mighty miracles, and especially by the greater miracle of his own resurrection from the dead. According to the spirit of holiness. It must be noted that this is a contrast with "according to the flesh" in Ro 1:3, and hence must refer to our Lord's holy nature. The body was descended from David, but the pure, holy life was demonstrated to be divine. One was a human nature; the other was a divine nature. This nature is spoken of as "the spirit of holiness", because it is contrasted with sinful flesh.