Augustine of Hippo

"Serve the Lord with fear;" lest what is said, "You kings and judges of the earth," turn into pride: "And rejoice with trembling" [Psalm 2:11]. Very excellently is "rejoice" added, lest "serve the Lord with fear" should seem to tend to misery. But again, lest this same rejoicing should run on to unrestrained inconsiderateness, there is added "with trembling," that it might avail for a warning, and for the careful guarding of holiness. …

Exposition on Psalm 2 ↗

Thomas Aquinas

Then when he says, "serve": after understanding, he fittingly sets forth service, because the service of God, which is latria, is the profession of faith. And therefore one must first believe, and afterward confess and serve: Rom. 10: "With the heart one believes unto justice, and with the mouth," etc. He says, moreover, "the Lord": for he who serves a man, it suffices that he be outwardly subject to him by obeying; but he who serves God must be inwardly subject to him according to the soul, having a good affection: Ps. …

Exposition on the Psalms of David ↗

Bonaventure

The fear of God avails for introducing the rectitude of divine justice. "The fear of the Lord drives out sin. For he who is without fear cannot be justified." Injustice does not enter the soul except through sin: but the first justification of the soul is that it be subject to the divine majesty. "If you do not hold yourself steadfastly in the fear of the Lord, your house will quickly be overturned." Consider David, who says: "Serve the Lord in fear, and rejoice in him with trembling."

Collationes de Septem Donis, Collation 2 ↗