“For if the inheritance is of the law, it is no longer of promise; but God gave it to Abraham by promise.”

For if the inheritance is of the law, it is no longer of promise;

God’s promise would no longer be unilateral but bilateral if it depended on people somehow. If God’s blessing depends on them as well as Him, it negates God’s grace. This would wipe out the very nature of a promise. The two ideas of law and grace are mutually exclusive. There is no intermediate position between them.

but God gave it to Abraham by promise

The word “gave” is a term of grace. The root for “gave” is the same as the word for “grace,” meaning to give graciously and generously, with the implication of goodwill on the giver’s part–to grant, to bestow. The Greek tense shows that God’s grace stands permanent (Greek perfect tense; the action completed in the past with results continuing).

Grace is free. God gives His promise with no strings attached. Since His character is at stake, He is the One who does the providing. We cannot measure up to God’s perfection, so He gives that perfection to us without charge.

Principle:

Grace and law exclude each other.

Application:

Man cannot improve on God’s promise. We do not earn or deserve the right to receive God’s grace. If the law could change grace at all, it would render it void. We cannot combine grace and law because there is no middle ground.

“I do not set aside the grace of God; for if righteousness comes through the law, then Christ died in vain [without a cause]” (Galatians 2:21).

Salvation and sanctification must rest on grace or law, but not both. They are two opposing principles. God glorifies Himself by what He does. His glory does not depend on us. God glorifies Himself by giving something.