Rom 3:4

“If one is to be righteous, one must keep the law in every particular, which is almost impossible for human nature to achieve. Therefore every one is a liar. For since every man is a liar, it follows that on that day when the Lord comes to judgment with men, only he will be justified in what he says. For his words are true in everything, because they are the words of truth.It should also be understood that this saying, i.e., that every man is a liar, is taken from Psalm 116:11 … Now someone might object that if all men are liars, then Paul too, being a man, will also be a liar! But in that case David, who originally said it, would also be a liar, and what he said would be false, just because he was a liar.… The whole statement would thus become nonsense, which is absurd!”

“Does God lie? Truly He lies not, because it is impossible for God to lie. And further, does this impossibility arise from infirmity? No, truly, for how can He be Almighty if He cannot do all things? What then is impossible to Him? Not that which is difficult to His Power, but what is contrary to His Nature. It is impossible, it is said, for Him to lie. This impossibility comes not of infirmity, but of Power and Majesty, for truth admits not of falsehood, nor God's Power of the weakness of error. Wherefore let God be true and every man a liar.”

“"Every man" means the majority. Paul's opponents had quoted this text as if David had meant: "For this reason have I sinned, that thou might appear just in judging me." But the true meaning is that God promised to punish sinners without showing favoritism and that some thought his delay in doing so amounted to a lie. God prevails when he judges the deeds of those from whom no one thought vengeance would ever be exacted. Or it may also mean that God has shown that he is concerned about mankind … and that he has overthrown those who wrongly suggest that he is not interested in human affairs.”

“Above he said that some did not believe. Yet it turns out that not just some, but all proved faithless. Therefore, so as not to grieve the Jews, he speaks wisely and presents what was shown by experience in the form of a supposition. Let us say, he says, that all were faithless. What of it? Even in this case God is justified. That is: if one considers and compares what God bestowed upon the Jews and how they conducted themselves before Him, then righteousness remains on God's side, as David also says (Ps. 50:6).”

“Then when he says, God forbid! But God is true, he shows that it is unfitting for God's faithfulness to be nullified on account of men's unbelief. First, he gives a reason for this; second, he cites a text, at as it is written; third, he excludes a false understanding of this text, at but if our injustice. The reason is based on the fact that God in himself is true: the Lord is the true God (Jer 10:10); this is the true God and life eternal (1 John 5:20); and every man a liar: I said in my consternation: men are all a vain hope (Ps 116:11). Hence, it is plain that man's mendacity or unbelief in not adhering to the truth does not nullify God's truth or faithfulness. This is easier to understand, if we consider that truth implies agreement between thing and understanding. But things are in agreement with our understanding in one way and with God's in another way. For our intellect derives its knowledge from things; consequently, the cause and measure of our truth stems from the thing's being. For an opinion is called true or false depending on whether the thing is as stated or is not. Hence, our understanding can be true or it can be false, for it can be in agreement or disagreement. But whatever is open to being or not being needs someone acting to make it be; otherwise, it continues not to be. For as air without something illuminating it remains dark, so our intellect by itself, unless it is enlightened by the first truth, continues in falseness. Hence, of himself every man is false in his intellect and is true only in virtue of participating in the divine truth: send out your light and your truth (Ps 43:3). The divine intellect, on the other hand, is the cause and measure of things. For this reason it is of itself unfailingly true, and everything else is true inasmuch as it conforms to that intellect. Similarly, considering truth on the part of the thing, man of himself does not have truth, because his nature is convertible into nothing. Only the divine nature, which is not produced from nothing or convertible into nothing, has of itself truth. Then when he says as it is written, he proves his statement on the authority of a text in a psalm: that you may be justified in your words and may overcome when you are judged (Ps 51:4). How this is to the point can be gathered from considering what the Psalmist had said just ahead of it. For he says just before this, against you, you only, have I sinned, and then: that you may be justified in your words and may overcome when you are judged. For God through the prophet Nathan had promised David that he would establish his kingdom forever in his seed (2 Sam 7:16). But later, when he fell into serious sin, namely, adultery and murder (2 Sam 11:2ff.), some said that on account of these sins God would not keep the promises made to him. Hence, the Psalmist's intention bears on two things. First, that God's justice, which involves keeping his promises, is not changed on account of sin. Touching on this he says, that you may be justified in your words, i.e., that you may be shown just in your words, since you do not disregard them because of my sins: all the words of my mouth are righteous (Prov 8:8); the Lord is faithful in all his words (Ps 145:13). Second, that God's promise imitates men's judgment. And this is what he says, and may overcome, namely, by keeping your promise, when you are judged, namely, by men, that on account of my sins you did not keep your promises: be not overcome by evil, but overcome evil by good (Rom 12:21), which is said to men. Accordingly, it is truer of God. It should be noted that God's promise to David was to be fulfilled in Christ's Incarnation. Hence it was a predestinative prophecy, in which something is promised as destined to be fulfilled in every way; whereas something promised or foretold by a prophecy of warning is not predicted as destined to be fulfilled in every way but according to men's merits, which can change. Therefore, if the promise made to David had not been fulfilled, it would have been prejudicial to God's justice; whereas the non-fulfillment of a promise made through a prophecy of warning is not prejudicial to God's justice, but indicates a change in human merits. Hence it is written: if at any time I declare concerning a nation or a kingdom that I will pluck up and break down and destroy it, and if that nation turns from its evil, I will repent of the evil that I intended to do to it (Jer 18:7). Therefore, it is plain, according to this sense, that man's sin does not exclude God's faithfulness. Other senses of this text are presented in the Gloss, but they are not closely related to the Apostle's intention. The first is that these words are linked with the words, wash me more thoroughly from my iniquity (Ps 50:4), to the end that you may be justified, i.e., appear just, in your words, in which you promised pardon to sinners not only in Ezekiel, because these words antedated that, but also in Leviticus: if they make amends for their iniquity, then I will remember my covenant (Lev 26:41), and Deuteronomy: if, led by repentance of heart, you turn to God, the Lord your God will bring you back, and have compassion on you (Deut 30:1). Thus you may overcome when you are judged by men that you ought not pardon me. Second these words are connected with the verse against you only have I sinned, i.e., in comparison with you who alone are just. And this is what he means when he says that you may be justified, i.e., may you appear just in comparison to me and other sinners: the Lord is just, and loves justice (Ps 10:8), and this not only in deeds but also in your words, which is the greatest of all according to James, if anyone does not offend in word (Jas 3:2). And may overcome when you are judged, i.e., when you are compared to anyone else in judgment: judge between me and my vineyard (Isa 5:3). Third, these words are referred to Christ, who alone is without sin: he committed no sin; no guile was found on his lips (2 Pet 2:22) and is, therefore, justified in his words as compared to all men. And overcome sin, death, and the devil: the Lion of the tribe of Judah has conquered (Rev 5:5), when you are judged unjustly by Pilate: your cause has been judged as that of the wicked (Job 36:17).”

“Because God is true, he gives what he has promised. To fail is human, for the times and the foolishness of nature make man unstable in that he does not have foreknowledge. But God, for whom there is no future, remains unchanging, as he says: "I the Lord do not change." Therefore Paul says that all men are liars, and this is true. For nature is fallible and is not unreasonably called a liar. It may be a liar intentionally or accidentally, but we must not expect God to be like that, for he is perfect and full of good will and will accomplish what he has promised. He even confirms this by the prophetic oracle: "Thou art justified in thy sentence and blameless in thy judgment."”