It was about His Son, Jesus Christ our Lord, who was born as a physical descendant of King David.

Even King David also spoke of the blessing experienced by the person who was considered righteous by God apart from doing good deeds.

And King David said [Psa. 69:22-23], "Let their table ensnare and trap them [i.e., let the Jews' devotion to such things as lavish feasts become the occasion for leading them astray], and become an occasion for their falling [away from God]. And let them be repaid [for such conduct with punishment].

and so that the Gentiles [also] may honor God for His mercy, as it is written [Psa. 18:49], "Therefore, I will praise you among the Gentiles and sing to your name." [Note: Paul here accommodates the words of King David by applying them to Jews participating with Gentiles in praising God as part of the one body].