4:21 Paul challenged his readers, who claimed to value the Law so highly, to consider what it taught. He chose his lesson from Genesis, a book in the "Law"section of the Old Testament. Thus he used the term "law"to refer to two different things in this verse: the Mosaic Law and the Old Testament. Again Paul returned to Abraham, the founder of Judaism.
4:22-23 He pointed out two contrasts between Ishmael and Isaac. First, Ishmael's mother was a slave, but Isaac's mother was free. These conditions affected the status of their sons in Abraham's household. Second, Ishmael was born naturally, but Isaac was born supernaturally in answer to God's promise.
"In the scriptural record of the birth of these two sons of Abraham Paul recognizes the same opposition between reliance on self (according to the flesh') and reliance on God (through promise') as exists between those who would be justified by legal works and those who are justified by faith."155
