Verses 21-31 provide a biblical illustration of law and grace. Paul produces a final rebuttal of legalism by giving an allegorical interpretation of Abraham and his difficulties with his two wives and two sons. In a masterful move, Paul turned to a scriptural illustration to conclude his theological defense of justification by faith. An Old Testament story from the life of Abraham enabled Paul to review what he had already declared about contrasts between the Mosaic Law and grace, between works and faith. It also provided an opportunity for him to verbalize the pointed charge to the Galatians that they should cast out the legalizers (v. 30).

The theme of Christian freedom that is especially prominent in the last two chapters is anticipated here by the contrast between Hagar the slave and Sarah the free woman.