15 “Let my heart be made glad, so that it may fear Your name.” There is then fear in gladness. How can there be gladness, if fear? Is not fear wont to be painful? There will hereafter be gladness without fear, now gladness with fear; for not yet is there perfect security, nor perfect gladness. If there is no gladness, we faint: if full security, we rejoice wrongly. Therefore may He both sprinkle on us gladness, and strike fear into us, that by the sweetness of gladness He may lead us to the abode of security; by giving us fear, may cause us not to rejoice wrongly, and to withdraw from the way.
Therefore says the Psalm: “Serve the Lord in fear, and rejoice unto Him with trembling:” so also says the Apostle Paul; “Work out your own salvation with fear and trembling; for it is God that works in you.” Whatever prosperity comes then, my brethren, is rather to be feared: those things which you think to be prosperous, are rather temptations. An inheritance comes, there comes wealth, there is an abundant overflow of some happiness: these are temptations: take care that they corrupt you not.
Whatever prosperity also there is according to Christ, and the true love of Christ: if perhaps you have gained your wife, who was of the party of Donatus: if your sons have been made believers who were Pagans: if perhaps you have gained your friend who wished to draw you away to the theatres, and you have drawn him to the church: if some hostile opponent of yours who was furiously mad against you, laying aside his fury, has become gentle, and owned God, and now barks at you no more, but cries with you against wickedness: these things are pleasant.
For what do we rejoice for, if we do not rejoice for these things? Or what other are our joys, but these? But because tribulations also abound, and temptations, and dissensions, and schisms, and other evils, without which this world cannot be, until iniquity pass away: let not that rejoicing make us secure, but let our heart be so made glad, as to fear the name of the Lord, lest it be made glad on one side, be stricken on another. Expect not security in journeying: if ever we wish for it here, it will be the birdlime of the body, not the safety of the man. “Let my heart be made glad, so that it may fear Your name.”
Source: The Enarrations, or Expositions, on the Psalms (New Advent)