Proverbs 16:6 By mercy and truth iniquity is purged: by the fear of the Lord men depart from evil.
Taken as an unconditional promise Jesus Christ is mercy and truth, and Christ purges us from sin. “The word became flesh…full of grace and truth….grace and truth came through Jesus Christ [John 1:14, 17].” Faith in Christ is the fear of the Lord, and faith justifies, or makes righteous, or causes evil to go away. Because of Christ all sin is purged (there is no need for purgatory), and all evil is departed. “The blood of Jesus his Son cleanses us from all sin [1 John 1:7].” This promise is true on account of Christ, given by grace, and received by faith.
If we should take this as a conditional promise the meaning is a little different. If we show mercy and speak truth, then sin will be purged from relationships. “As we forgive those who trespass against us,” and “Love covers a multitude of sins.” If we believe in Jesus (fear the Lord), then we will depart from evil, that is, faith will cause us not to sin so much. We may confidently pray. “Deliver us from evil.” Believe the promise: the evil is separated from us. Of course, we remember that mercy, love, and forgiveness is not some thing can actually do. God loves through us. And fearing God (believing) is not something sinners can actually do either. Faith (fear) itself is a gift of God.
So let us take the promise both ways: unconditional and conditional, spiritually and practically, justification and sanctification, we are forgiven as we forgive. God loves me, and through me He loves others, thereby effectively combatting sin and evil.