This persecution of Isaac by Ishmael sprang from an enmity which always exists between the world and the people of God. Christ warned his disciples, ‘If the world hate you, ye know that it hated me before it hated you. If ye were of the world, the world would love his own: but because ye are not of the world, but I have chosen you out of the world, therefore the world hateth you’ (John 15:18-19). The reason the world hated Christ is that he testified that its works are evil. The unbeliever’s conscience reminds him that a day is coming when he will have to give account to God for all his sins, and on that day he will be condemned. For the same reason the world hates the Christian, for it recognises that those who belong to Christ have an inheritance which is denied them.
This assault on the gospel by the Judaizers Paul interprets as a form a persecution, for although it did not involve physical violence towards property or person, it was an assault on what was most precious of all – the truth which God had given them for the salvation of their souls. In Paul’s case this opposition had often resulted in physical persecution so that he had no doubt where it would lead if these enemies of the truth did not get their way. Persecution takes various forms, but the enmity behind it remains the same. While the Judaizers hoped to win the Galatians over to their side, they were ready to use argument and persuasion, but the time would come when that approach was abandoned and they would oppose the believers more forcefully as they had opposed Paul.
Isaac is said here to be born of the Spirit, but this is not a reference to the new birth of John 3, for this birth was physical and is in contrast to Ishmael’s natural birth. This is called a birth of the Spirit because it was by the promise of God and by the miraculous intervention of the Spirit of God that Isaac was born when any natural birth was beyond hope due to the advanced years of his parents. But although the birth referred to here is physical, it is certainly a type of the new birth, and the contrast between Ishmael and Isaac is between the two types of man that exist on the earth, the natural man and the spiritual man. Isaac’s physical birth by the Spirit’s power did not confer spiritual life on him but only physical, nevertheless he must have been born again subsequently, for Isaac was a man of faith and without the new birth there can be no faith.