A Commentary by John Stott
Verse 28: *Now we, brethren, like Isaac are children of promise*. If we are Christians, we are like Isaac, not Ishmael. Our descent from Abraham is spiritual not physical. We are not his sons by nature, but by supernature.
What follows is this: If we are like Isaac, we must expect to be treated as Isaac was treated. The treatment Isaac got from his half-brother Ishmael is the treatment that Isaac’s descendants will get from Ishmael’s descendants. And the treatment that Isaac got from his father Abraham is the treatment that we must expect from God.
a). We must expect persecution.
Verse 29: *But as at that time he who was born according to the flesh persecuted him who was born according to the Spirit, so it is now.* At the ceremony at which Isaac was weaned, when he was probably a boy of three and Ishmael a youth of seventeen, Ishmael ridiculed his little half-brother Isaac. We do not know the details of what happened, because Ishmael’s attitude is described by only one Hebrew verb, probably meaning that he ‘laughed’ or ‘mocked’ (Gn.21:9). Nevertheless, it is clear that Isaac was the object of Ishmael’s scorn and derision.
We must expect the same. The persecution of the true church, of Christian believers who trace their spiritual descent from Abraham, is not always by the world, who are strangers unrelated to us, but by our half-brothers, religious people, the nominal church. It has always been so. The Lord Jesus was bitterly opposed, rejected, mocked and condemned by His own nation. The fiercest opponents of the apostle Paul, who dogged his footsteps and stirred up strife against him, were the official church, the Jews. The monolithic structure of the medieval papacy persecuted all Protestant minorities with ruthless, unremitting ferocity. And the greatest enemies of the evangelical faith today are not unbelievers, who when they hear the gospel often embrace it, but the church, the establishment, the hierarchy. Isaac is always mocked and persecuted by Ishmael.
b). We shall receive the inheritance.
Verse 30: *But what does the Scriptures say? ‘Cast out the slave and her son; for the son of the slave shall not inherit with the son of the free woman.’*. Although Isaac had to endure the scorn of his half-brother Ishmael, it was Isaac who became heir of his father Abraham and received the inheritance. At one stage Abraham wanted Ishmael to be the heir: ‘Oh that Ishmael might live in thy sight!’ he cried to God. And God replied, ‘No,…But I will establish my covenant with Isaac’ (Gn.17:18-21). So Sarah asked Abraham to cast out the slave and her son, and God told Abraham to do what Sarah said. For, although he was going to make a nation of the slave woman’s son too (that is Ishmael the father of the Arabians), yet He added ‘through Isaac shall your descendants be named’ (Gn.21:10-13).
So it is that the true heirs of God’s promise to Abraham are not his children by physical descent, the Jews, but his children by spiritual descent, Christian believers whether Jews or Gentiles. And since it is ‘the Scripture’ which said ‘Cast out the slave and her son’, we find the law itself rejecting the law. This verse of Scripture, which the Jews interpreted as God’s rejection of the Gentiles, Paul boldly reverses and applies to the exclusion of unbelieving Jews from the inheritance. As J.B.Lightfoot comments, ‘the Apostle thus confidently sounds the death-knell of Judaism.’
Such, then, is the double lot of ‘Isaacs’ – the pain of persecution on the one hand and the privilege of inheritance on the other. We are despised and rejected by men; yet we are the children of God, ‘and if children, then heirs, heirs of God and fellow heirs with Christ’ (Rom.8:17). This is the paradox of a Christian’s experience. As Paul puts it in 2 Corinthians 6:8-10, we are ‘in honour and dishonour, in ill repute and in good repute,…as sorrowful, yet always rejoicing; as poor, yet making many rich; as having nothing, and yet possessing everything.