Based on Psalm 51, the best-known of the penitential psalms, “God, Be Merciful” is a collation from the complete versification of the psalm in the 1912 Psalter.
Bert Polman, Psalter Hymnal Handbook
God’s children are not called to come before God’s throne with a list of accomplishments, or merits or goodness; they are called, says Our World Belongs to God, paragraph 26, to come with the humility that “…offers nothing but our need for mercy.” Such a cry for mercy comes from our “dying-away of the old self” (Heidelberg Catechism, Lord’s Day 33, Question and Answer 88) which expresses that we are “genuinely sorry for our sin and more and more…hate and run away from it” (Heidelberg Catechism, Lord’s Day 33, Question and Answer 89).
The gifts of renewal and pardon come only “through true faith” (Heidelberg Catechism, Lord’s Day 7, Question and Answer 20) and are “gifts of sheer grace, granted solely by Christ’s merits” (Heidelberg Catechism, Lord’s Day 7, Question and Answer 21). The very act of faith is to plead for his mercy.