Summary
When Jesus was declared son of God through the resurrection, it showed that He is more than just a descendant of David in a physical sense—He is God as well. So Jesus is fully God yet also fully man.
Content
What does it mean for Jesus to be “declared Son of God”? Basically, it means that Jesus was and is fully God, with the resurrection serving as proof positive.
Romans 1:3 says that Jesus was “a descendant of David according to the flesh”, and Romans 1:4 says that Jesus was “declared Son of God” via the resurrection. He was more than just a descendant of David according to the flesh, in other words.
Together, these concepts provide two guideposts in our understanding of Christ. Various heresies in the past have fallen into a ditch upon either side of this road. On the one hand, certain gnostics de-emphasized Jesus’ humanity, viewing flesh as inherently evil. (This is true in a general sense for humans with a sin nature, but not Christ, who was born without a sin nature. This is, in fact, a sometimes-overlooked reason why the virgin birth is so important theologically). On the other hand, Arians questioned the divinity of Jesus.
Going too far either direction is problematic. Jesus is fully God and fully man, both. This doctrine is sometimes termed “the hypostatic union”. It is one of those things (alongside God’s infinity and externality to spacetime, for example) that is difficult for us as humans to fully understand. But we need to accept it on faith, making peace with the fact that God is bigger and greater than our human minds can comprehend. Atheists may scoff at this so-called “God of the gaps”, but God has structured this world to require faith from us for a reason (that is, He has intentionally arranged the world so that we need to exercise faith in order to properly navigate it), and this matter of Jesus being somehow fully God yet also at the same time fully man is no exception.