Albert Barnes Commentary Psalms 13:5

"But I have trusted in thy lovingkindness; My heart shall rejoice in thy salvation." — Psalms 13:5 (ASV)

But I have trusted in your mercy - In your favor; your friendship; your promises. His original confidence had been in God only, and not in himself. That confidence he still maintained; and now, as the result of that, he begins to exult in the confidence that he would be safe. The idea is, “I have trusted in the mercy of God; I still trust, and I will trust forever.”

My heart shall rejoice in your salvation - The word “salvation” here does not refer to salvation in the future world, but to deliverance from his present troubles, or to God’s interposition in putting him into a condition of safety. The idea is, that he had entire confidence that God would interpose, and that there would yet be cause to rejoice in that salvation as actually accomplished. He now calls on his heart to rejoice in the assurance that it would be his. So with us. There will not only be rejoicing in salvation when actually accomplished, but there may, and should be, in the firm conviction that it will be ours.