For David is not ascended into the heavens; hence St. Peter here proves, that these words, spoken by David, were not principally to be understood concerning himself, but concerning Christ the Messiah; for David, as to his body, was in the sepulchre, which on that account was kept amongst them.
The Lord said unto my Lord; the eternal Father unto his eternal Son, who was now made flesh—hence our Saviour proves his Divinity, Mat_22:45. The words here referred to are Psa_110:1.