John Calvin Commentary Psalms 89:24

"But my faithfulness and my lovingkindness shall be with him; And in my name shall his horn be exalted." — Psalms 89:24 (ASV)

My truth and my mercy shall be with him. God shows that He will continue to exercise without interruption that grace which He had first shown to David. These words are as if He had said that, to prove Himself faithful to His word, He would always be gracious and bountiful. Thus, we see that God not only provided David with testimonies of His goodness at the beginning, but He also always continued to deal with him in the same merciful way.

This has a reference to the whole Church of Christ, so that divine goodness is manifested in the whole course of our salvation, and not only at our first entrance into it, as these evasive and sophistical Sorbonists foolishly claim. The horn of David denotes here, as it often does elsewhere, his glory, dignity, and power. Therefore, the meaning is that by the grace of God, this kingdom shall always flourish and prosper.