Romanos 9:8-9

‘That is, it is not the children of the flesh who are children of God, but the children of the promise are reckoned for a seed. For this is a word of promise, “According to this season will I come, and Sarah will have a son”.

For the conclusion to be reached from the facts of Scripture is that it is not the children of the flesh who are children of God, but the children of promise, in other words those foreknown of God (Romanos 11:1), and chosen by Him. And he gives, as an example of God's promises, the promise that Sarah would have a son ‘when He (God) came' (Génesis 18:10). ‘When He came' indicated that the son of promise would be miraculously born to aged parents. So it should be noted that the promise related to a child especially elected by God, produced as a result of the activity of God, and being but a portion of the whole, an indication of what would follow.

‘The children of the promise.' In Gálatas 4:28 ‘the children of promise' are those who are ‘born after the Spirit' rather than the flesh (Gálatas 4:29), that is by the miraculous working of God, and this because they are the result of God acting in accordance with His own promise and determination (Gálatas 4:23). In the same way in Romans the usual parallel with flesh is the life producing Spirit (Romanos 8:4), and this ties in with the idea here that ‘God will come' to Sarah at the right time, that is, will visit her in order to bring about a miraculous birth, and will do it according to the word of promise. It was God Who, outside the normal scheme of things, determined that Isaac would be born. Thus the idea behind ‘the children of the promise' is of those born supernaturally in accordance with God's promise and determination. In other words they are exceptionally born through God's foreknowing (Romanos 8:29) and through the Spirit (consider Juan 3:1). Indeed when God says, ‘I will come' it always indicates divine activity as in Juan 14:23 (compare Juan 14:18), and Lucas 1:68 (compare Lucas 1:35).