When God gave his promise to Abraham, since there was no one greater by whom he could swear, he swore by himself.
This was the oath which he swore to our forefather Abraham--
To take an illustration, Brothers, from daily life--No one sets aside even an agreement between two men, when once it has been confirmed, nor does he add conditions to it.
If our heritage is the result of Law, then it has ceased to be the result of a promise. Yet God conferred it on Abraham by a promise.