4 “Unto the right-hearted there arises up light in the darkness”. Justly do the godly direct their heart unto their God, justly do they walk with their God, preferring His will to themselves; and having no proud presumption in their own. For they remember that they were some time in darkness, but are now light in the Lord. “Merciful, pitying, and just is the Lord God.” It delights us that He is “merciful and pitying,” but it perhaps terrifies us that the Lord God is “just.”
Fear not, despair not at all, happy man, who fearest the Lord, and hast great delight in His commandments; be thou sweet, be merciful and lend. For the Lord is just in this manner, that He judges without mercy him who has not shown mercy; but, “Sweet is the man who is merciful and lends”: God will not spew him out of His mouth as if he were not sweet. “Forgive,” He says, “and you shall be forgiven; give, and it shall be given unto you.” Whilst you forgive that you may be forgiven, you are merciful; while you give that it may be given unto you, you lend.
For though all be called generally mercy where another is assisted in his distress, yet there is a difference where you spend neither money, nor the toil of bodily labour, but by forgiving what each man has sinned against you, you gain free pardon for your own sins also....He who is unwilling to give to the poor, seeks riches; listen to what is written, “You shall have treasure in heaven.” You will not then lose honour by forgiving: for it is a very laudable triumph to conquer anger. You will not grow poor by giving; for a heavenly treasure is a more safe possession. The former verse, “Riches and plenteousness shall be in his house,” was pregnant with this verse.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)