St Jerome, commenting on Galatians 3:11-12, says:
We should note that he did not say that just any man lives by faith, lest he provide an excuse for the devaluation of virtuous deeds. Rather, he said that the righteous man lives by faith. This means that before having faith and the intention to live by it, one must already be righteous and must by the purity of his life have claimed certain steps that lead to faith. It is therefore possible for someone to be righteous without yet living by faith in Christ. If this is troublesome to the reader, let him consider what Paul says about himself [Phil 3:6]: "As for righteousness according to the Law, I was faultless." At the time, Paul was righteous in terms of keeping the Law, but he was not yet able to live by faith because he did not have ChristThis is astonishing, because with this explanation, Paul is basically undermining the very erroneous Protestant idea which teaches that our own sinfulness prevents our works from saving us. In Jerome's explanation, a person who is already righteous still needs faith. This fits perfectly with what I've written about numerous times (e.g. Here and Here) against the Protestant heresy of Salvation By Good Works Alone, contrasted to the Catholic teaching of St Paul which is Salvation by Faith.
10 For all who rely on works of the law are under a curse; for it is written, “Cursed be everyone who does not abide by all things written in the Book of the Law, and do them.” 11 Now it is evident that no one is justified before God by the law, for “The righteous shall live by faith.” 12 But the law is not of faith, rather “The one who does them shall live by them.”By citing Habakkuk 2:4 within the context of the "works of the law," per Jerome's insights this means that Paul has in mind particularly the Jew-Gentile controversy, and is thus saying even if a Jew were following the Mosaic Law, and thus were 'righteous per the Law', such is not enough. Faith would still be needed. Jerome further elaborates that Paul mentions "live" in two instances here: faith causes life and keeping the law causes life. Since both cannot be true in the same sense must mean that the "life" that the Law gives is a temporal living, such as long life and earthly blessings, as well as avoiding the death penalty that the Law holds over a person for grave violations of the Mosaic Covenant. Meanwhile, the "life" that faith in Christ brings is eternal life.