3 Concerning His Son Jesus Christ our Lord, which was made of the seed of David according to the flesh;
4 and declared to be the Son of God with power, according to the Spirit of holiness, by the resurrection from the dead.
Paul packs a lot into these two short verses.
Concerning — Refers back to the gospel of God in verse 1. The gospel concerns Jesus Christ.
His Son Jesus Christ our Lord — God’s Son (therefore, God) Jesus Christ (referring to His humbling and rejection as well as His exaltation) our Lord (referring to His position as sovereign over us).
Which was made of the seed of David according to the flesh — This was in fulfillment of the Old Testament prophecies that from David would come the Messiah/King (2 Samuel 7:13; Jeremiah 33:17). It also shows Jesus Christ’s humanity. He was made (literally “born”), becoming something He wasn’t — human.
And declared to be the Son of God — This shows His deity. He was made to be human, but He was declared to be God. The Greek word translated “declared” means “marked by unmistakable signs.” The same sense of “made human” and “declared to be God” can be seen in Isaiah 9:6 — For unto us a Child is born, unto us a Son is given. And again in Galatians 4:4 — But when the fullness of the time had come, God sent forth His Son, born of a woman, born under the law. As a human, Jesus Christ was made, born of a woman, born in the flesh. As God, He was declared, given, and sent by the Father.
With power — With the power of God. It was in God’s power that Jesus Christ was made human and declared to be God.
According to the Spirit of holiness — There is some question whether the “Spirit” in this verse refers to the Holy Spirit or Christ’s own spirit, but either way it doesn’t change the meaning. Christ was holy — sinless. Romans 1:4 describes two things that prove beyond doubt that Jesus Christ is God. The first is His sinless holiness. Everyone born of man is sinful. Christ was born of the Spirit and is holy.
By the resurrection from the dead — This is the second proof that Jesus Christ is God. Death is the result of sin. Jesus Christ was sinless. He volunteered to take our sins, but because of His own sinlessness and His deity, His resurrection was His Divine prerogative, not a result of Divine favor. The resurrection from the dead doesn’t just refer to His own resurrection. Because He rose and lives, He can raise us up to life after our death — yet more proof that Jesus Christ is God.