Psalm 21:7 For the king trusteth in the Lord; and through the mercy of the most high he shall not be moved.
Because God is so great and so good to him, because God has blessed him forever, and because God has made him exceeding glad, David the king trusts in the Lord. And through the mercy of the Lord he shall not be moved. David responds to God’s great salvation, grace, and blessing with trust. He has saved me from my enemies before, and I am most secure in the knowledge that He will do it again.
Just as the Lord has saved David from his enemies and granted mercy and blessing, so also the Lord has done the same thing for us by the mighty deliverance from and defeat of our spiritual enemies, sin, death, and the devil. God did this once and for all at the Cross and the Tomb. And He continues to grant the blessings of that great salvation every day. We respond to this wonderful Gospel with trust. The Spirit uses this gospel to create a secure faith in our heart every time we hear it.
The outcome of faith in His mercy is that we shall not be moved. Nothing in all the world or under the world can shake us from that position in which we stand. Lies, doubts, and accusations will assault us daily, but we will not be moved off that solid foundation. The promise is sure, and we believe it. Trust is our response to grace. “God said it; I believe it; and that settles it for me.”