Paul decides to come into the discussion about the law by bringing Abraham (the father of the nation) into play. Issac was the son God promised, but Ishmael (the son by the slave woman) was the son that Abraham produced of his own accord. Ishmael was not the son of the promise, but he was still a son. God’s working out of the covenant works the same way: the law was a framework, it was a glimpse into what the fullness of the covenant could be, just like Ishmael as a son was a glimpse of what having a son could be. But it was not the end of the story. The fullness of the covenant came with Jesus, just as the fullness of God’s covenant came after Ishmael, with Isaac.