Psalm 13:5

But I have trusted in thy mercy
The faith, hope, and comfort of the psalmist grew and increased by prayer; from complaining he goes to praying, from praying to believing; he trusted not in himself, not in his own heart, nor in his own righteousness and merits, but in the mercy of God; and not in the bare absolute mercy of God, but in the grace and goodness of God, as the word ^F24 here used signifies, as it is displayed in the plenteous redemption which is by Christ; which is a sufficient ground of faith and hope; see ( Psalms 130:7 ) ;

my heart shall rejoice in thy salvation;
which God is the contriver, author, and giver of, and in which the glory of his perfections is so greatly displayed: and a true believer rejoices more on account that God is glorified by it than because of his own interest in it; and this joy is an inward one, it is joy in the heart, and is real and unfeigned, and is what continues, and will be felt and expressed both here and hereafter.