A.T. Robertson Commentary Romans 1:3

"concerning his Son, who was born of the seed of David according to the flesh," — Romans 1:3 (ASV)

Concerning his Son (περ του υιου αυτου). Just as Jesus found himself in the O.T. (Luke 24:27,46). The deity of Christ here stated.

According to the flesh (κατα σαρκα). His real humanity alongside of his real deity. For the descent from David see Mt 1:1,6,20; Luke 1:27; John 7:42; Acts 13:23, etc.