15 Therefore follows: “The Lord will commend His loving-kindness in the day-time; and in the night-time will He declare it”. In tribulation no man has leisure to hear: attend, when it is well with you; hear, when it is well with you; learn, when you are in tranquillity, the discipline of wisdom, and store up the word of God as you do food. For in tribulation every one must be profited by what he heard in the time of security. For in prosperity God “commends to you His mercy,” in case thou serve Him faithfully, for He frees you from tribulation; but it is “in the night” only that He “declares” His mercy to you, which He “commended” to you by day.
When tribulation shall actually come, He will not leave you destitute of His help; He will show you that which He commended to you in the daytime is true. For it is written in a certain passage, “The mercy of the Lord is seasonable in the time of affliction, as clouds of rain in the time of drought.” “The Lord has commended His loving-kindness in the day-time, and in the night will He declare it.” He does not show that He is your Helper, unless tribulation come, from whence you must be rescued by Him who promised it to you “in the day-time.”
Therefore we are warned to be like “the ant.” For just as worldly prosperity is signified by “the day,” adversity by the night, so again in another way worldly prosperity is expressed by “the summer,” adversity by the winter. And what is it that the ant does? She lays up in summer what will be useful to her in winter. Whilst therefore it is summer, while it is well with you, while you are in tranquillity, hear the word of the Lord. For how can it be that in the midst of these tempests of the world, you should pass through the whole of that sea, without suffering?
How could it happen? To what mortal's lot has it fallen? If even it has been the lot of any, that very calm is more to be dreaded. “The Lord has commended His loving-kindness in the day-time, and in the night-time will He declare it.”...“There is with me prayer unto the God of my life.” This I make my business here; I who am the “hart thirsting and longing for the water-brooks,” calling to mind the sweetness of that strain, by which I was led on through the tabernacle even to the house of God; while this “corruptible body presses down the soul,” there is yet with me “prayer unto the God of my life.”
For in order to making supplication unto God, I have not to buy anything from places beyond the sea; or in order that He may hear me, have I to sail to bring from a distance frankincense and perfumes, or have I to bring “calf or ram from the flock.” There is “with me prayer to the God of my life.” I have within a victim to sacrifice; I have within an incense to place on the altar; I have within a sacrifice wherewith to propitiate my God. “The sacrifice of God is a troubled spirit.” What sacrifice of a “troubled spirit” I have within, hear.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)