concerning his Son, who was descended from David according to the flesh (Romans 1:3)
First, Paul identities Christ Jesus as God’s Son (Ro 1:1, 3). He is divine and heavenly. Then, Paul identifies him as a descendent of David according to the flesh. He is equally human. He is completely and thoroughly God while also completely and thoroughly man.
Perhaps more to Paul’s point, Jesus fulfills the promise God made to David long before, when he said:
When your days are fulfilled and you lie down with your fathers, I will raise up your offspring after you, who shall come from your body, and I will establish his kingdom. He shall build a house for my name, and I will establish the throne of his kingdom forever. (2 Samuel 7:12, 13)