Here Paul points out who Christ is. Paul shows that the Old Testament prophecy is fulfilled with Christ, by pointing out He was a descendant of King David. This also says that Christ was truly Human and was of Royal decent. Then Paul goes into the Divinity of Christ. With "the Spirit of holiness" Christ is the Son of God by being raised from death. Through Christ we receive grace and are called to be apostles.

Romans 1:3-5