13 “The Lord is full of compassion and mercy: long-suffering, and of great mercy”. Why so long-suffering? Why so great in mercy? Men sin and live; sins are added on, life continues: men blaspheme daily, and “He makes His sun to rise over the good and the wicked.” On all sides He calls to amendment, on all sides He calls to repentance, He calls by the blessings of creation, He calls by giving time for life, He calls through the reader, He calls through the preacher, He calls through the innermost thought by the rod of correction, He calls by the mercy of consolation: “He is long-suffering, and of great mercy.”
But take heed lest by ill using the length of God's mercy, you store up for yourself, as the Apostle says, wrath in the day of wrath....For some there are who prepare to turn, and yet put it off, and in them cries out the raven's voice, “Cras! Cras!” The raven which was sent from the ark, never returned. God seeks not procrastination in the raven's voice, but confession in the wailing of the dove. The dove, when sent forth, returned. How long, Tomorrow! Tomorrow!? Look to your last morrow: since you know not what is your last morrow, let it suffice that you have lived up to this day a sinner. You have heard, often you are wont to hear, you have heard today also; daily you hear, and daily you amend not....
Source: The Enarrations, or Expositions, on the Psalms (New Advent)