‘Now it was not written for his sake alone, that it was reckoned to him, but for our sake also, unto whom it will be reckoned, who believe on him who raised Jesus our Lord from the dead,'
And this is now true also for all his spiritual sons. That faith was reckoned for righteousness was not just written for Abraham's sake, it was written also for ‘our sakes' (for the sake of true believing Christians). For in the same way as faith was reckoned for righteousness in Abraham's case, faith will also be reckoned for righteousness in the case of all those who believe on Him Who raised Jesus our Lord from the dead. It would appear here that Paul is seeing the birth of Isaac by a miracle, as being like a foretaste of the miracle of the resurrection. Both would result in a multitude of progeny.