5 For Moses writes about the righteousness which is of the law, “The man who does those things shall live by them.”
The paragraph running from verse 5 to 13 contrasts two kinds of righteousness:
Righteousness which has its origin in the law (the law provides no means for fulfilling its demands).
Righteousness which comes from faith in Christ.
Verses 5-8 describe the implications of a person who tries to fulfill the law by works.
The “for” here gives the basis for the assertion in verse 4. The idea of 10:4 was that Christ and Christ alone fulfilled all the demands of the law. Any other attempt to gain salvation would be utterly vain.
Paul does not quote directly from the Old Testament in the next sequence of verses. He merely puts the teaching of salvation by faith and grace in Scriptural language.
Moses writes about the righteousness which is of the law,
Moses was the author of the first five books of the Bible. Those books establish the law under the economy of the nation Israel. Jews dispersed to Rome would understand the implication of these verses from the Old Testament.
“The man who does those things [the law] shall live by them.”
This sentence is an allusion to Leviticus 18:5. Should a Jew attempt to keep the standards of the law by human achievement he would utterly fail. The words “shall live by them” indicate that the Jew would have to rest his case before God by keeping the entire law. This is something that the Jew cannot attain because of the absolute character of God. No one can measure up to His righteousness.
Ga 3:12 Yet the law is not of faith, but “the man who does them shall live by them.”
The Old Testament sacrificial system taught the proper way of salvation. It was by atonement, by substitutional sacrifice. These sacrifices only pointed to the One who would fully paid for sins. The type cannot save but only the antitype. A symbol could never give eternal life to anyone.
The person who chooses to go to heaven by keeping the law must keep them in every respect.
A person who chooses to attain heaven by works would have to keep the entire law perfectly to be saved. This is an unbearable task. The issue is keeping all the law, not most of it. Since people cannot meet the demands of the law, they need a Savior. They need grace, not works. Salvation by works means doing everything the law teaches.
Ga 3: 10 For as many as are of the works of the law are under the curse; for it is written, “Cursed is everyone who does not continue in all things which are written in the book of the law, to do them.”
Ja 2: 10 For whoever shall keep the whole law, and yet stumble in one point, he is guilty of all.
If we chose to be saved through the law then God holds us accountable for everything, every commandment that is in the law. If a person fails in just “one” point of the law then God will judge for failing all the law. That is why no one can be justified before the law:
Ga 3: 11 But that no one is justified by the law in the sight of God is evident, for “the just shall live by faith.”