ANSWER: The sure mercies of David are mentioned twice: "Incline your ear, and come unto Me: hear, and your soul shall live; and I will make an everlasting covenant with you, even the sure mercies of David." Isa. 55:33Incline your ear, and come unto me: hear, and your soul shall live; and I will make an everlasting covenant with you, even the sure mercies of David. (Isaiah 55:3). "And as concerning that He raised Him up from the dead, now no more to return to corruption, he said on this wise, I will give you the sure mercies of David. Wherefore He saith also in another psalm, Thou shalt not suffer Thine Holy One to see corruption." Acts 13:34, 3534And as concerning that he raised him up from the dead, now no more to return to corruption, he said on this wise, I will give you the sure mercies of David. 35Wherefore he saith also in another psalm, Thou shalt not suffer thine Holy One to see corruption. (Acts 13:34‑35).
It is evident from these scriptures that the sure mercies of David are connected with Christ in resurrection. Isa. 53 gives us the rejection of Christ by Israel, His substitutionary work, His resurrection and exaltation; then chapter .54 gives some of the results for Israel, and chapter 55 exhorts and encourages on this groundwork; hence the "sure mercies of David" come in here.
In Acts 13 the same theme is discovered, for the previous verse, the 33rd, tells us that God fulfilled the promises made to the fathers when He raised up Jesus (not "again," but raised Him up by sending Him into this world- incarnation) according to the word in the "second psalm, Thou art My Son, this day have I begotten Thee." Then in the 34th verse the Holy Spirit speaks of His being raised up from the dead, "now no more to return to corruption," and quotes Isa. 55:33Incline your ear, and come unto me: hear, and your soul shall live; and I will make an everlasting covenant with you, even the sure mercies of David. (Isaiah 55:3) about the sure mercies of David to substantiate the unchangeableness of His resurrection and consequent blessing.
The 16th psalm is also called upon to render testimony to this truth: "Thou shalt not suffer Thine Holy One to see corruption."
There is a greater connection between "Thine Holy One" of the 35th verse and the "sure mercies of David" of the 34th verse than appears on the surface. They are practically the same word in the original language. Psalm 89 is a case in point. There in the first verse the psalmist says he will sing of the mercies of Jehovah, and in the 19th verse, "Thou spakest. to [or, of] Thy holy one." The only difference in the Hebrew text between "mercies" of the first verse and "holy" of the 19th verse is that the first is plural and the second singular. Thus all the mercies of God are centered in His Son, the holy, or merciful One. The same connection is to be found in Acts 13 between the "Holy One" of verse 35 and the "mercies" of verse 34.
Therefore, we can conclude that the mercies to Israel based on a Messiah coming through the seed of David were lost when they cried "away with Him" and chose "a murderer and a robber." But God, who foresaw all, promised them "sure" mercies through Him in whom all mercies center, but in Him risen and glorified. So if Israel rejected Christ, and with Him their own blessing, God will bring all to pass without fail in Him whom He raised from the dead, and who will never return to it. The promises of blessing to David through his heir were not sure in the sense that when they came (in the Person of Christ) they were rudely rejected, but they are made sure in Him-David's greater Son -in resurrection. "For all the promises of God in Him are yea, and in Him Amen"; or, they are affirmed in Him and will all be confirmed in Him (2 Cor. 1:2020For all the promises of God in him are yea, and in him Amen, unto the glory of God by us. (2 Corinthians 1:20)).