5 What then does the Lord say? “Serve ye the Lord in fear, and rejoice unto Him with trembling.” So the Apostle too, “Work out your own salvation with fear and trembling. For it is God who works in you.” Therefore rejoice with trembling: “Lest at any time the Lord be angry.” I see that you anticipate me by your crying out. For you know what I am about to say, you anticipate it by crying out. And whence have ye this, but that He taught you to whom you have by believing come?
This then He says; hear what ye know already; I am not teaching, but in preaching am calling to your remembrance; nay, I am neither teaching, seeing that you know already, nor calling to remembrance, seeing that you remember, but let us say all together what together with us ye retain. “Embrace discipline, and rejoice,” but, “with trembling,” that, humble ye may ever hold fast that which you have received. “Lest at any time the Lord be angry;” with the proud of course, attributing to themselves what they have, not rendering thanks to Him, from whom they have.
“Lest at any time the Lord be angry, and you perish from the righteous way.” Did he say, Lest at any time the Lord be angry, and you come not into the righteous way? Did he say, “Lest the Lord be angry, and He bring you not to the righteous way”? Or “admit you not into the righteous way? You are walking in it already, be not proud, lest ye even perish from it. 'And ye perish,' says he, 'from the righteous way.'” “When His wrath shall be kindled in a short time” against you. At no distant time. As soon as you are proud, you lose at once what you had received. As though man terrified by all this were to say, “What shall I do then?” It follows, “Blessed are all they that trust in Him:” not in themselves, but in Him. “By grace are we saved, not of ourselves, but it is the gift of God.”
Source: Sermons on Selected Lessons of the New Testament (New Advent)