Psalms 13:5

But I have trusted in thy mercy; my heart shall rejoice in thy salvation.

The psalm was probably written by David, but it is not his song alone; it is an inspired song given to God's children and proper to sing and study at all times. It stands in the first of the five books of the Psalms, part of God's great treasure-house in the Psalms, helping to impress the truth of God's Fatherhood and the spiritual truths revealed in the first five books of Moses and in Job. The psalm begins with a sense of great need and ends with triumph and confidence in God, trusting mercy and rejoicing in salvation, pointing to a God who is able and willing and anxious to save and who gives help in discouragement, strength in trial, and solid comfort in grief. That trust and assurance of divine help and final deliverance stands amid the sorrows and afflictions of the Hebrew captives during the Babylonian captivity.

Trust in God's mercy is the reason the heart rejoices in His salvation, and this is first joy, joy in God because of His mercy toward sinners. This is unlike rejoicing in riches, fame, pleasure, and what man has done, which apart from God is folly and but for a moment, for those who rejoice in such things rejoice in a thing of naught. The joy comes through God's mercy, which reconciles sinners to Him as Father.

The heart rejoices because Christ dwells in the believer giving the hope of glory, and because God hears and answers prayer and forgives and saves from sin. As love is from God, so is joy; love holds all good things, so whatever brings love to the heart brings joy and wherever love is exercised joy is found.

In the verse, trust is wondrous personal love and salvation is a full and free gift already tasted. The heart rejoices in Jesus and sings to the Lord, asking mercy to guard and keep the believer worthy of the great reward though weak and having done little to gain love. The believer continues to strive in obedience to God's word for a home above.