When Christ was born into the world, he had to largely lay aside his divinity and his visible power. He had to do that because he had to be as a man with human weakness; not a human sinful nature, but with human weaknesses, made in the likeness of sinful flesh.
‘According to the spirit of holiness.’ Some translators put the word Spirit with a capital ‘S’ in there, referring to the Holy Spirit. Our translators in the Authorised Version have taken the view that verse 4 applies entirely to Jesus Christ. If they are correct, what it means is this: Paul is saying is that Christ appeared to be a man, but by the resurrection, by the exercise of his own divinity and power he demonstrated who he truly was, the eternal Son of God. So the ‘spirit of holiness’, with a small ‘s’, refers here to his own innate power and Godhead. So the verse refers to Christ raising himself. But Scripture also tells us that God the Father raised him, and God the Holy Spirit raised him. So we put all these Scriptures together, and we say that because it was the will of the one God, all members of the one Godhead were at work in the resurrection.