Romans 4:14,15

‘For if those who are of the law are heirs, faith is made void, and the promise is made of none effect. For the law works wrath; but where there is no law, neither is there transgression.'

The promises were offered to Abraham for his ready acceptance through faith. Thus faith was the basis of his heirdom. That being so, if that heirdom goes to those who rely on observing the Law for salvation, faith is basically cancelled out. It is no longer required. It is rendered ineffectual, being replaced by law-keeping. And the consequence of that is that the promise which was offered to faith would also have been made of none effect. This would be so because those attempting to keep the Law would inevitably fail to fully keep the Law (as described in Chapter s 1-3). Thus they will be under wrath. For the Law works wrath, that is, it makes men's sins specific and thus multiplies them. And a holy God will not fulfil His promises to those who are under His wrath. Compare Galatians 3:10, ‘cursed are all those who do not continue in the book of the Law to do them' (cited from Deuteronomy 27:26). On the other hand because there was no Mosaic Law in the time of Abraham, those who lived then would not come under the constant wrath that resulted from the continual breaking of ‘the Law', as it did not then exist. They would be ‘without transgression', that is, not guilty of breaking the Law of Moses. The idea is not that they were sinless. It is that their approach to God was not based on Law but on faith.