Ps 39:11

“"Remove not Thou Thy mercies far from me, O Lord" [Psalm 40:11]. He is turning his attention to the wounded members. Because I have not "concealed Thy mercy and Thy Truth from the great congregation," from the Unity of the Universal Church, look Thou on Thy afflicted members, look on those who are guilty of sins of omission, and on those who are guilty of sins of commission: and withhold not Thou Thy mercies. "Thy mercy and Thy Truth have continually preserved me." I should not dare to turn from my evil way, were I not assured of remission; I could not endure so as to persevere, if I were not assured of the fulfilment of Thy promise. ...”

“"But you, O Lord." Above, the Psalmist commemorated the confidence he has in God with regard to past things; here, however, he presents the confidence he has in God with regard to future things, expressing it through prayer. And concerning this he does two things. First, he proposes the prayer in general. Second, in particular, at "May it please you." Concerning the first he does two things. First, he proposes the petition. Second, he assigns the reason for petitioning, at "Your mercy." He says therefore, "I have proclaimed your justice." "But you, O Lord, do not put your compassions far from me." Some already receive the compassion of God, namely those who enjoy grace and glory: Ps. 118: "The earth is full of the mercy of the Lord." Others are near, who have no impediment to mercy but some merit: Ps. 144: "The Lord is near to all who call upon him." Others are those in whom there is an impediment; therefore mercy is far from them. And therefore I ask that your compassion not be far from me on account of my demerits. And therefore the reason for this petition is twofold: one of confidence, which was conceived from the experience of divine goodness; the other of necessity, which he suffers. There are two things according to which God comes to our aid: namely mercy, insofar as he bestows upon us certain things that he has not even promised: Rom. 15: "That the Gentiles might glorify God for his mercy"; and truth, when he renders either what was promised or what is due for merits. And therefore these are united together: because he always does what he has promised, or does even more abundantly. These two are necessary for a person: mercy, so that he may be drawn to the hope of glory; and truth, so that he may be confident. And this is not as though we draw God to ourselves by our merits, but God through these things draws and has drawn us. And therefore he says, "Your mercy and your truth have always sustained me."”