Romans 4:16

‘For this reason it is of faith, that it may be according to grace, to the end that the promise may be sure to all the seed, not to that only which is of the law, but to that also which is of the faith of Abraham, who is the father of us all.'

It is because the Law can only bring down on men the wrath of God that God's promise had to be based on faith, so that the promise could depend on the unfailing grace of God. This alone made the promise sure of fulfilment. And it was a fulfilment that would be available to ‘all the seed', that is all whom God had promised to bless through Abraham (the whole world - Genesis 12:3). But that being so, this sureness of fulfilment was now not just to be seen as available to those who were ‘of the Law', if they believed, but was also to be seen as available towards all who believed God as Abraham believed God. And this was because the Scriptures say that Abraham is ‘the father of us all', not just of those who called themselves the sons of Abraham.