If the Law did the job, what would happen to the promise?
If the Law did the job, the promise would no longer be the basis; it would be nullified. The inheritance would come by Law instead of promise. But the Law could not fulfill any promises, only condemn Israel, so God's Covenant remained based on promise.
Les explains from verse 18 that if the inheritance were of the Law, then it is no more based on promise. If the Law was going to do the job, then it is no more a promise. God gave Abram promise after promise—'I will, I will, I will'—and the Law could not fulfill any of those promises. All the Law could do was condemn Israel. It did not fulfill any promises per se. But God gave the Covenant based on promise. Later, Les notes the Law was added 2,500 years after Adam, and about 430 years (roughly 500) after Abraham, showing it was added for a reason, not to replace the promise.
For if the inheritance be of the law, it is no more of promise: but God gave it to Abraham by promise.
More Questions
What happens to the promise if the inheritance comes by the Law?
If the inheritance is by the Law, then it is no longer based on promise.