12 All iniquity, be it little or great, punished must needs be, either by man himself repenting, or by God avenging. For even he that repents punishes himself. Therefore, brethren, let us punish our own sins, if we seek the mercy of God. God cannot have mercy on all men working iniquity as if pandering to sins, or not rooting out sins. In a word, either you punish, or He punishes....
Source: The Enarrations, or Expositions, on the Psalms (New Advent)