3 “And enter not into judgment with Your servant”. Who are willing to enter into judgment with Him, save they who, “being ignorant of the righteousness of God, go about to establish their own?” “Wherefore have we fasted, and You have not seen; wherefore have we afflicted our souls, and You take no knowledge?” As though they would say, “We have done what You have commanded, why do You not render to us what You have promised?” God answers you: I will give to you to receive what I have promised: I have given you that you should do that whereby you may receive.
Finally, to such proud ones the Prophet speaks; “Wherefore will you plead with Me? You have all transgressed against Me, says the Lord.” Why will you enter into judgment with Me, and recount your own righteousnesses?...“For before You every one living shall not be justified.” “Every one living;” living, that is, here, living in the flesh, living in expectation of death; born a man; deriving his life of man; sprung from Adam, a living Adam; every one thus living may perhaps be justified before himself, but not before You.
How before himself? By pleasing himself, displeasing You. Enter not then into judgment with me, O Lord my God. How straight soever I seem to myself, You bring forth a standard from Your store-house, Thou fittest me to it, and I am found crooked. Well is it said, “with Your servant.” It is unworthy of You to enter into judgment with Your servant, or even with Your friend....What of the Apostles themselves?...That ye may perceive it at once, they learned to pray what we pray: to them was given the pattern of prayer by the heavenly Counsellor.
“After this manner,” says He, “pray ye.” And having set down certain things first, He laid down this too to be said by the leaders of the sheep, the chief members of the Shepherd and Gatherer of the one flock; even they learned to say, “Forgive us our debts.” They said not, “Thanks be to You, who hast forgiven us our debts, as we too forgive our debtors,” but, “Forgive, as we forgive.” But surely the faithful prayed then, surely the Apostles prayed then, for this Lord's Prayer was given rather to the faithful.
If those debts only were meant which are forgiven by Baptism, it would befit catechumens rather to say, “Forgive us our debts.” Let the Apostles then say, yea let them say, “Forgive us our debts.” And when it is said to them, “Wherefore say ye this? What are your debts?” let them answer, “for in Your sight every one living shall not be justified.”
Source: The Enarrations, or Expositions, on the Psalms (New Advent)