4:3 what saith the scripture. The Scripture cited is Genesis 15:6. As Paul notes later (Romans 4:9-15), Abraham was justified by faith before he was given the sign of circumcision (Genesis 17:9-14) and before the law was given, so justification by faith is God’s original and basic standard (that is, of course, a genuine faith in God and His Word, a faith which then causes one to obey God’s Word).

4:22 therefore. Note the cause-and-effect relation between strong faith in God’s promises and His imputation of Christ’s righteousness to us.