28 Now you, brothers and sisters, like Isaac, are children of promise. 29 At that time the son born according to the flesh persecuted the son born by the power of the Spirit. It is the same now. 30 But what does Scripture say? “Get rid of the slave woman and her son, for the slave woman’s son will never share in the inheritance with the free woman’s son.” 31 Therefore, brothers and sisters, we are not children of the slave woman, but of the free woman.
Paul pursues the illustration still further. In the record of the animosity and shown by Hagar and Ishmael against Isaac, he sees another expression of a fundamental principle. Not only is there a contrast between the two sons, there is conflict. The one opposes the other. The one always, invariably, resents the other. Not only in the story of Ishmael, but in the history of Ishmael's posterity has there been that irreconcilable opposition on their part against God's elect. So also in the days of the New Testament, the two seeds of Abraham, the natural and the spiritual, are set in opposition, in the Baptist's day, in Christ's ministry - they finally crucified Him - and in the early Church alike. We have only to think of the terrible persecution engineered by the Pharisees against the Apostles in Acts to realise the truth of this. And what of Paul himself, before his conversion, 'breathing out threatenings and slaughter against the disciples of the Lord'. This is inevitable. It lies in the nature of the case that there will always be this antagonism. This is why it will always be an uphill fight to establish the truth of the gospel in the midst of error. But the truth of the gospel, acceptance by faith rather than works is ultimately vindicated by God Himself. What does the Scripture say about this conflict? 'Cast out the bondwoman and her son.' This is the divine verdict upon the contention. These are chosen, those are left and rejected. Such is the import of Paul's words at the close of the chapter.