21 “Therefore God has hearkened to me”. Because I have not beheld iniquity in my heart. “And He has listened to the voice of my prayer.” “Blessed be my God, that has not thrust away my supplication and His mercy from me”. Gather the sense from that place, where he says, “Come ye, hear, and I will tell you, all you that fear God, how great things He has done to my soul:” he has both said the words which you have heard, and at the end thus he has concluded: “Blessed be my God, that has not thrust away my supplication and His mercy from me.”
For thus there arrives at the Resurrection he that speaks, where already we also are by hope: yea both it is we ourselves, and this voice is ours. So long therefore as here we are, this let us ask of God, that He thrust not from us our supplication, and His mercy, that is, that we pray continually, and He continually pity. For many become feeble in praying, and in the newness of their own conversion pray fervently, afterwards feebly, afterwards coldly, afterwards negligently: as if they have become secure.
The foe watches: you sleep. The Lord Himself has given commandment in the Gospel, how “it behooves men always to pray and not to faint.” And he gives a comparison from that unjust judge, who neither feared God, nor regarded man, whom that widow daily importuned to hear her; and he yielded for weariness, that was not influenced by pity: and the naughty judge says to himself, “Though neither God I fear, nor men I regard, even because of the weariness which this widow daily puts upon me, I will hear her cause, and will avenge her.”
And the Lord says, “If a naughty judge has done this, shall not your Father avenge His chosen, that to Him do cry day and night? Yea, I say unto you, He shall make judgment of them speedily.” Therefore let us not faint in prayer. Though He puts off what He is going to grant, He puts it not away: being secure of His promise, let us not faint in praying, and this is by His goodness. Therefore he has said, “Blessed is my God, that has not thrust away my supplication and His mercy from me.” When you have seen your supplication “not thrust away from you,” be secure, that His mercy has not been thrust away from you.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)