Because the law works wrath: for where no law is, there is no transgression.
All Commentaries on Romans 4:15 Go To Romans 4
John Chrysostom
AD 407
The law works wrath and makes those who are under it liable for their transgressions, which is a curse, not a promise! … But when faith comes it brings grace with it, and so the promise takes effect. For where there is grace there is forgiveness, and where there is forgiveness there is no punishment. Once punishment is removed and righteousness takes hold from faith, there is no obstacle to our becoming heirs of the promise.