Acts 2:34-35

For David is not ascended into the heavens - That is, David has not risen from the dead and ascended to heaven. This further shows that Psalms 16:1 could not refer to David, but must refer to the Messiah. Great as they esteemed David, and much as they were accustomed to apply these expressions of the Scripture to him, yet they could not be applicable to him. They must refer to some other being; and especially that passage which Peter now proceeds to quote. It was of great importance to show that these expressions could not apply to David, and also that David bore testimony to the exalted character and dignity of the Messiah. Hence, Peter here adduces David himself as affirming that the Messiah was to be exalted to a dignity far above his own. This does not affirm that David was not saved, or that his spirit had not ascended to heaven, but that he had not been exalted in the heavens in the sense in which Peter was speaking of the Messiah.

But he saith himself - Psalms 110:1.

The Lord - The small capitals used in translating the word “Lord” in the Bible denote that the original word is יהוה Yahweh. The Hebrews regarded this as the unique name of God, a name incommunicable to any other being. It is not applied to any being but God in the Scriptures. The Jews had such a reverence for it that they never pronounced it; but when it occurred in the Scriptures they pronounced another name, אדני ̀Adonaay. Here it means, “Yahweh said,” etc.

My Lord - This is a different word in the Hebrew - it is אדני ̀Adonaay. It properly is applied by a servant to his master, or a subject to his sovereign, or is used as a title of respect by an inferior to a superior. It means here, “Yahweh said to him whom I, David, acknowledge to be my superior and sovereign.” Thus, though he regarded him as his descendant according to the flesh, yet he regarded him also as his superior and Lord. By reference to this passage our Saviour confounded the Pharisees, Matthew 22:42. That the passage in this Psalm refers to the Messiah is clear. Our Saviour, in Matthew 22:42, expressly applied it thus, and in such a manner as to show that this was the well-understood doctrine of the Jews. See the notes on Matthew 22:42, etc.