My covenant will I not break, nor {b} alter the thing that is gone out of my lips.

(b) For God in promising has respect to his mercy, and not to man's power in performing.

Matthew Henry's Concise Commentary

89:19-37 The Lord anointed David with the holy oil, not only as an emblem of the graces and gifts he received, but as a type of Christ, the King Priest, and Prophet, anointed with the Holy Ghost without measure. David after his anointing, was persecuted, but none could gain advantage against him. Yet all this was a faint shadow of the Redeemer's sufferings, deliverance, glory, and authority, in whom alone these predictions and promises are fully brought to pass. He is the mighty God. This is the Redeemer appointed for us, who alone is able to complete the work of our salvation. Let us seek an interest in these blessings, by the witness of the Holy Spirit in our hearts. As the Lord corrected the posterity of David for their transgressions, so his people shall be corrected for their sins. Yet it is but a rod, not a sword; it is to correct, not to destroy. It is a rod in the hand of God, who is wise, and knows what he does; gracious, and will do what is best. It is a rod which they shall never feel, but when there is need. As the sun and moon remain in heaven, whatever changes there seem to be in them, and again appear in due season; so the covenant of grace made in Christ, whatever alteration seems to come to it, should not be questioned.