NET
18 For if the inheritance is based on the law, it is no longer based on the promise, but God graciously gave it to Abraham through the promise.
Darby
18 For if the inheritance [be] on the principle of law, [it is] no longer on the principle of promise; but God gave it in grace to Abraham by promise.
KJV
18 For if the inheritance [be] of the law, [it is] no more of promise: but God gave [it] to Abraham by promise.