The law itself is not opposed to faith (see 3:19–25; Romans 7:7–13), but trying to be righteous by following the law opposes righteousness by faith in Christ. Paul quotes Leviticus 18:5 to show that life under the law of Moses requires obedience rather than belief. It is impossible to be accepted by God by trying to obey the law of Moses perfectly (Galatians 3:10–11).