‘For it is written that Abraham had two sons, one by the handmaid and one by the free-woman. But the son by the handmaid is born after the flesh, while the son by the free-woman is born through promise.'
For the Law itself indicates two ways, one the way of freedom, and the other the way of bondage. Just as Abraham had had two sons, one born under freedom, and one born under bondage.
‘It is written.' Indicating the word of God given through Scripture. And what does the Scripture tell us? It tells us that of Abraham's two sons one was born of the handmaid by human choice following fleshly aims and desires, and was born in the normal way, ‘of the flesh', but the other was born in God's will as a result of the specific promise of God, as the child ‘of promise', and was born miraculously.
Thus the lesson of the Law is that it is possible to be a son of Abraham by human fleshly descent and it is possible to be a son of Abraham as a result of being born supernaturally through promise. The former was the boast of the Judaisers (and all Jews). They claimed that they were, by human descent, sons of Abraham. (The fact that this was not true for many was lost in the mists of time). But if they would only realise it this simply associated them with Ishmael.
But then there are other sons. They are sons through promise and through the miraculous working of God. These Paul will tell us represent those who have responded to the promise of God offered in the Gospel.
And it is this basic idea that then leads on to the application of the two covenants, the covenant of promise and the covenant of works (or of the flesh), to the mothers of these two types of son, by allegory.
‘Born after the flesh -- born through promise.' Ishmael was the result of human planning and manipulation. He was basically the product of unbelief. But Isaac was promised beforehand by God and came in accordance with that promise and all the promises that had gone before which would apply to him. He was the child of promise. And it was through believing in these promises that Abraham had been reckoned as righteous by faith. That occurred because he believed God's promises. Thus Abraham's blessings came as a result of faith in God's promises, a faith which resulted in his being reckoned as righteous (Galatians 3:8), and not as a result of his fleshly activity, planned and wrought by the flesh, when he produced Ishmael.