Paul must have realized that his readers would find it difficult to understand why faith and law are two different ways and why only faith, not law, leads to righteousness (acceptance by God). So he sets up a third signpost that repeats the antithesis between faith and law and supports that antithesis by a quotation from Leviticus 18:5 regarding the nature of the law: The law is not based on faith; on the contrary, "The man who does these things will live by them."

The fundamental nature of law is that it requires doing. When Paul refers to law here, he cannot mean the whole of the Pentateuch (the five books of Moses). The whole of the Pentateuch (law in the broad sense) is primarily concerned with faith in God. Paul has already quoted from Genesis 15:6, which describes the way of faith exemplified by Abraham. In Galatians 3:12 law must be taken in the narrow sense as a reference to the specific divine requirements given to the Jewish people through Moses. In the context of Leviticus 18 the things to be done are the "decrees and laws" God gave Israel at Mount Sinai so that the Israelites would be distinguished from the Egyptians and Canaanites (Lev 18:1-4). In the Galatian dispute, the law refers to a set of requirements (specifically circumcision, food laws and sabbath laws) imposed on Gentile believers which would identify them with the Jewish nation and set them apart from Greeks and Romans.

Paul is not making an abstract, absolute contrast between believing and doing. His rebuke is aimed at the folly of doing the works of the law as a means of participating in the life and blessing of the covenant people of God. The law is not of faith, because it demands doing the works of the law as the way to life, whereas it has just been demonstrated (v. 11) that righteousness by faith is the way to life. The law demands perfect obedience (v. 10) and offers life on the basis of this perfect obedience (v. 12), but in itself the law is incapable of imparting life or righteousness before God (v. 21). So Paul puts up a stop sign in front of those who want to follow the law as the way to life. You can't get to life that way. Life is found only through faith in Christ.