12 But this thing is not granted to believing Jews alone....“The eyes of Him do look upon the Gentiles.” And what do we? The Jews will murmur; the Jews will say, “what He has given to us, the same to them also; to us Gospel, to them Gospel; to us the Grace of Resurrection, and to them the Grace of Resurrection; does it profit us nothing that we have received the Law, and that in the justifications of the Law we have lived, and have kept the commandments of the fathers? Nothing will it avail? The same to them as to us.” Let them not strive, let them not dispute. “Let not them that are bitter be exalted in their own selves.” O flesh miserable and wasting, are you not sinful? Why cries out your tongue? Let the conscience be listened to. “For all men have sinned, and need the glory of God.” Know yourself, human weakness. You received the Law, in order that a transgressor also of the Law you might be: for you have not kept and fulfilled that which you received. There has come to you because of the Law, not the justification which the Law enjoins, but the transgression which you have done. If therefore there has abounded sin, why do you envy Grace more abounding. Be not bitter, for “let not them that are bitter be exalted in their own selves.” He seems in a manner to have uttered a curse in “Let not them that are bitter be exalted;” yea, be they exalted, but not “in themselves.” Let them be humbled in themselves, exalted in Christ. For, “he that humbles himself shall be exalted; and he that exalts himself shall be humbled.” “Let not them that are bitter be exalted in their own selves.”
13. “Bless our God, you nations”. Behold, there have been driven back they that are bitter, reckoning has been made with them: some have been converted, some have continued proud. Let not them terrify you that grudge the Gentiles Gospel Grace: now has come the Seed of Abraham, in whom are blessed all nations. Bless ye Him in whom you are blessed, “Bless our God, you nations: and hear ye the voice of His praise.” Praise not yourselves, but praise Him. What is the voice of His praise? That by His Grace we are whatever of good we are. “Who has set my Soul unto life” Behold the voice of his praise: “Who has set my Soul unto life.” Therefore in death she was: in death she was, in yourself. Thence it is that you ought not to have been exalted in yourselves. Therefore in death she was, in yourself: where will it be in life, save in Him that said, “I am the Way, the Truth, and the Life”? Just as to certain believers the Apostle says, “You were sometime darkness, but now light in the Lord.”...“And has not given unto motion my feet.” He has set my Soul unto life, He guides the feet that they stumble not, be not moved and given unto motion; He makes us to live, He makes us to persevere even unto the end, in order that for everlasting we may live....
14. “For you have proved us, O God; You have fired us as silver is fired”. Have not fired us like hay, but like silver: by applying to us fire, You have not turned us into ashes, but You have washed off uncleanness, “You have fired us, as silver is fired.” And see in what manner God is angry against them, whose Soul He has set unto life. “You have led us into a trap:” not that we might be caught and die, but that we might be tried and delivered from it. “You have laid tribulations upon our back.” For having been to ill purpose lifted up, proud we were: having been to ill purpose lifted up, we were bowed down, in order that being bowed down, we should be lifted up for good. “You have laid tribulations on our back:” “You have set men over our heads”. All these things the Church has suffered in sundry and various persecutions: She has suffered this in Her individual members, even now does suffer it. For there is not one, that in this life could say that he was exempt from these trials. Therefore there are set even men over our heads: we endure those whom we would not, we suffer for our betters those whom we know to be worse. But if sins be wanting, a man is justly superior: but by how much there are more sins, by so much he is inferior. And it is a good thing to consider ourselves to be sinners, and thus endure men set over our heads: in order that we also to God may confess that deservedly we suffer. For why do you suffer with indignation that which He does who is just? “You have laid tribulations upon our back: You have set men over our heads.” God seems to be angry, when He does these things: fear not, for a Father He is, He is never so angry as to destroy. When ill you live, if He spares, He is more angry. In a word, these tribulations are the rods of Him correcting, lest there be a sentence from Him punishing....
15. “We have passed through fire and water.” Fire and water are both dangerous in this life. Certainly water seems to extinguish fire, and fire seems to dry up water. Thus also these are the trials, wherein abounds this life. Fire burns, water corrupts: both must be feared, both the burning of tribulation and the water of corruption. Whenever there is adversity, and anything which is called unhappiness in this world, there is as it were fire: whenever there is prosperity, and the world's plenty flows about one, there is as it were water. See that fire burn you not, nor water corrupt....Hasten not to the water: through fire pass over to the water, that you may pass over the water also. Therefore also in the mystic rites and in catechising and in exorcising, there is first used fire. For whence ofttimes do the unclean spirits cry out, “I burn,” if that is not fire? But after the fire of Exorcism we come to Baptism: so that from fire to water, from water unto refreshment. But as in the Sacraments, so it is in the temptations of this world: the straitness of fear draws near first, in place of fire; afterwards fear being removed, we ought to be afraid lest worldly happiness corrupt. But when the fire has not made you burst, and when you have not sunk in the water, but hast swum out; through discipline you pass over to rest, and passing over through fire and water, you are led forth into a place of refreshment. For of those things whereof the signs are in the Sacraments, there are the very realities in that perfection of life everlasting....But we are not torpid there, but we rest: nor though it be called heat, shall we be hot there, but we shall be fervent in spirit. Observe that same heat in another Psalm: “nor is there any one that hides himself from the heat thereof.” What says also the Apostle? “In spirit fervent.” Therefore, “we have gone over through fire and water: and You have led us forth into a cool place.”
Source: The Enarrations, or Expositions, on the Psalms (New Advent)