Matthew Henry Commentary Psalms 18:20-28

"Jehovah hath rewarded me according to my righteousness; According to the cleanness of my hands hath he recompensed me. For I have kept the ways of Jehovah, And have not wickedly departed from my God. For all his ordinances were before me, And I put not away his statutes from me. I was also perfect with him, And I kept myself from mine iniquity. Therefore hath Jehovah recompensed me according to my righteousness, According to the cleanness of my hands in his eyesight. With the merciful thou wilt show thyself merciful; With the perfect man thou wilt show thyself perfect; With the pure thou wilt show thyself pure; And with the perverse thou wilt show thyself froward. For thou wilt save the afflicted people; But the haughty eyes thou wilt bring down. For thou wilt light my lamp: Jehovah my God will lighten my darkness." — Psalms 18:20-28 (ASV)

Those who forsake the ways of the Lord depart from their God. But even though we are conscious of many false steps, let us not wickedly depart from our God. David kept his eye upon the rule of God's commands. Constant care to keep from that sin, whatever it may be, which most easily besets us, proves that we are upright before God.

Those who show mercy to others, even they need mercy. Those who are faithful to God will find him to be everything he has promised to be to them. The words of the Lord are pure words, very sure to be depended on, and very sweet to be delighted in. Those who resist God and walk contrary to him will find that he will walk contrary to them (Leviticus 26:21–24). The gracious recompense of which David spoke may generally be expected by those who act from right motives.

Hence he speaks comfort to the humble and terror to the proud; Thou wilt bring down high looks. And he speaks encouragement to himself; Thou wilt light my candle: you will revive and comfort my sorrowful spirit; you will guide my way, so that I may avoid the snares laid for me. You will light my candle to work by and give me an opportunity of serving you. Let those who walk in darkness and labour under discouragements take courage; God himself will be a Light to them.