3:11 just shall live by faith. In defense of his thesis, Paul not only argues from Genesis but also from the prophets, here quoting from Habakkuk 2:4 (see also Romans 1:17 and Hebrews 10:38). Martin Luther made this great verse, with its doctrine of justification by faith, the watchword of the Reformation.

3:12 that doeth them. This citation is from Leviticus 18:5, again reminding the Christians that if they were determined to submit to Jewish legalism, they would be obligated not only to assent to the law but to do it, and do all of it. This could earn salvation for them, theoretically, if they could do all the laws, but they could not. Only Christ could fulfill all the law (Matthew 5:17), so it is imperative that we receive His righteousness by imputation, and this can only be received through faith.