Romans 1:4b

and who through the Spirit of holiness was declared with power to be the Son of God by his resurrection from the dead: Jesus Christ our Lord.

(v4, NIV)

This messiah who was sent forth by God was raised from the dead. The mere fact that the Messiah died was quite devastating to the Jews. In their minds, they could not picture the Messiah to ever die and to die the way Jesus did.

but we had hoped that he was the one who was going to redeem Israel. And what is more, it is the third day since all this took place.

(Luke 24:21, NIV)

After Jesus is raised from the dead, he meets two people traveling a few miles away from Jerusalem. These two are sad because of the fact that Jesus died. At this point, they are kept from recognizing that it is Jesus who meets them. We read here that they had hoped that Jesus would redeem Israel. However, his death proved to them that Jesus was not the Messiah and that Jesus failed. We know that Jesus accomplished what he came to accomplish. However, it was difficult for them to accept a crucified Messiah.

And if the Spirit of him who raised Jesus from the dead is living in you, he who raised Christ from the dead will also give life to your mortal bodies through his Spirit, who lives in you.

(Romans 8:11, NIV)

Though we may not be exactly sure here in chapter 1, we can look to chapter 8 to confirm that it is the Spirit who raised Jesus from the dead. But what does it mean by the "Spirit of holiness"? Apparently, "Spirit of holiness" is only mentioned here in the bible. John Piper suggests that it is to ward away any sort of dirtiness and filthiness with regards to the dead. It may be hard for me to imagine that at first, since I always picture Jesus so awesome and holy and bright and white in his resurrected state. However, to those who do not know Jesus, they may be put off by the fact that we worship one who was dead for three days and rose from the dead. Some may picture zombies and witchcraft and demons and the devil. In order to ward away any notion of any filthiness with regards to Jesus' resurrection, the Spirit is referred to as the Spirit of holiness. Though dealing with the dead is dirty business, the resurrection of Jesus was done with holiness.