VERSE 18. For if the inheritance be of the law, it is no more of promise.
In Romans 4:14, the Apostle writes: "For if they which are made of the law be heirs, faith is made void, and the promise made of none effect." It cannot be otherwise. That the Law is something entirely different from the promise is plain. The Law thunders: "Thou shalt, thou shalt not." The promise of the "seed" pleads: "Take this gift of God." If the inheritance of the gifts of God were obtained by the Law, God would be a liar. We would have the right to ask Him: "Why did you make this promise in the first place: 'In thy seed shall all the nations of the earth be blessed'? Why did you not say: 'In thy works thou shalt be blessed'?"
VERSE 18. But God gave it to Abraham by promise.
So much is certain, before the Law ever existed, God gave Abraham the inheritance or blessing by the promise. In other words, God granted unto Abraham remission of sins, righteousness, salvation, and everlasting life. And not only to Abraham but to all believers, because God said: "In thy seed shall all the nations of the earth be blessed." The blessing was given unconditionally. The Law had no chance to butt in because Moses was not yet born. "How then can you say that righteousness is obtained by the Law?"
The Apostle now goes to work to explain the province and purpose of the Law.