But why would it not have been enough to name one of them, David alone, or Abraham alone? Because the promise had been made to both of Christ to be born of their seed. To Abraham, “And in thy seed shall all the nations of the earth be blessed.” [Gen 22:18] To David, “Of the fruit of thy body will I set upon thy seat.” [Ps 132:11]
He therefore calls Christ the Son of both, to shew that in Him was fulfilled the promise to both. Also because Christ was to have three dignities; King, Prophet, Priest; but Abraham was prophet and priest; priest, as God says to him in Genesis, “Take an heifer;” [Gen 15:9] Prophet, as the Lord said to Abimelech concerning him, “He is a prophet, and shall pray for thee.” [Gen 20:7] David was king and prophet, but not priest.
Thus He is expressly called the son of both, that the threefold dignity of His forefathers might be recognized by hereditary right in Christ.
Reproduced in full. Translation: Oxford 1841 (Newman), public domain (Public domain).