4:2 justified by works. The supposed contradiction between Paul and James has been widely noted. Paul says Abraham was not justified by works; James says he was (James 2:21). The point is that Abraham was justified by faith in God’s Word, but he then was asked to demonstrate that his faith was genuine by his works. He was justified before God by faith but was justified before men by his works (see James 2:21-24).
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).
4:7 Saying. This quote is from Psalm 32:1-2. This was David’s psalm of thanksgiving after his repentance over his notorious sin of adultery and homicide. Paul thus notes that justification by faith was true both before and after Moses—before, in Abraham, Israel’s great patriarch, and after, in David, Israel’s greatest king.
4:8 not impute sin. By the marvelous provision of imputation, our sins were debited to the account of Jesus, the Son of man, whereas His perfect righteousness was credited to our account. “For He hath made him to be sin for us, who knew no sin, that we might be made the righteousness of God in Him” (II Corinthians 5:21). See also James 2:23; Philemon 17-18.
4:16 father of us all. Abraham is the father not only of the Jews, physically speaking, but of all who are justified by faith, spiritually speaking. “They which are of faith, the same are the children of Abraham” (Galatians 3:7).