18 “I have not concealed Your mercy and Your Truth from the great congregation.” Let us be there; let us also be numbered among the members of this Body: let us not keep back “the mercy” of the Lord, and “the Truth” of the Lord. Would you hear what “the mercy of the Lord” is? Depart from your sins; He will forgive your sins. Would you hear what “the truth” of the Lord is? Hold fast righteousness. Your righteousness shall receive a crown. For mercy is announced to you now; “Truth” is to be shown unto you hereafter. For God is not merciful in such a way as not to be just, nor just in such a way as not to be merciful. Does that mercy seem to you an inconsiderable one? He will not impute unto you all your former sins: you have lived ill up to this present day; you are still living; this day live well; then you will not “conceal” this “mercy.” If this is meant by “mercy,” what is meant by “truth”?...
Source: The Enarrations, or Expositions, on the Psalms (New Advent)