10 “Lay hold of discipline, lest at any time the Lord be angry, and you perish from the righteous way”. This is the same as, “understand,” and, “be instructed.” For to understand and be instructed, this is to lay hold of discipline. Still in that it is said, “lay hold of,” it is plainly enough intimated that there is some protection and defence against all things which might do hurt unless with so great carefulness it be laid hold of. “Lest at any time the Lord be angry,” is expressed with a doubt, not as regards the vision of the prophet to whom it is certain, but as regards those who are warned; for they, to whom it is not openly revealed, are wont to think with doubt of the anger of God. This then they ought to say to themselves, let us “lay hold of discipline, lest at any time the Lord be angry, and we perish from the righteous way.” Now, how “the Lord be angry” is to be taken, has been said above. And “you perish from the righteous way.” This is a great punishment, and dreaded by those who have had any perception of the sweetness of righteousness; for he who perishes from the way of righteousness, in much misery will wander through the ways of unrighteousness.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)