How can God be merciful and still give justice?

Psalm 62 discusses how mercy belongs to God: 'Also unto thee, O Lord, belongeth mercy; for thou renderest to every man according to his work' (Psalm 62:12). This might appear contradictory, but it is through Christ's sacrifice that God's attributes of mercy and justice are harmonized. When believers are judged, they are considered righteous because they are in Christ, who fulfilled the law on their behalf. As such, God can extend mercy without compromising His justice, as Christ has borne the penalty for sin.