16 “The Lord is near unto all that call upon Him”. Where then is that, “Then shall they call upon Me, and I will not hear them”? See then what follows: “all who call upon Him in truth.” For many call upon Him, but not in truth. They seek something else from Him, but seek not Himself. Why do you love God? “Because He has made me whole.” That is clear: it was He that made you so. For from none else comes health, save Him. “Because He gave me,” says another, “a rich wife, whereas I before had nothing, and one that obeys me.” This too He gave: you say true. “He gave me,” says another, “sons many and good, He gave me a household, He gave me all good things.” Do you love Him for this?...Therefore if God is good, who has given you what you have, how much more blessed will you be when He has given you Himself! You have desired all these things of Him: I beseech you desire of Him Himself also. For these things are not truly sweeter than He is, nor in any way are they to be compared to Him. He then who preferrs God Himself to all the things which he has received, whereat he rejoices, to the things he has received, he “calls upon God in truth.”...
17. “He will perform the will of them that fear Him”. He will perform it, He will perform it: though He perform it not at once, yet He will perform it. Certainly if therefore you fear God, that you may do His will, behold even He in a manner ministers to you; He does your will. “And He shall hear their prayer, and save them.” You see that for this purpose the Physician hears, that He may save. When? Hear the Apostle telling you. For we are saved in hope: but hope which is seen is not hope: but if what we see not we hope for, then do we with patience wait for it: “the salvation,” that is, which Peter calls “ready to be revealed in the last time.”
18. “The Lord guards all that love Him, and all sinners He will destroy”. You see that there is severity with Him, with whom is so great sweetness. He will save all that hope in Him, all the faithful, all that fear Him, all that call upon Him in truth: “and all sinners He will destroy.” What “all sinners,” save those who persevere in sin; who dare to blame God, not themselves; who daily argue against God; who despair of pardon for their sins, and from this very despair heap up their sins; or who perversely promise themselves pardon, and through this very promise depart not from their sins and impiety? The time will come for all these to be separated, and for the two divisions to be made of them, one on the right hand, the other on the left; and for the righteous to receive the everlasting Kingdom, the wicked to go into everlasting fire. Since this is so, and we have heard the blessing of the Lord, the works of the Lord, the wondrous things of the Lord, the mercies of the Lord, the severity of the Lord, His Providence over all His works, the confession of all His works; observe how He concludes in His praise, “My mouth shall speak the praise of the Lord, and let all flesh bless His holy Name for ever and ever”.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)