How is being under grace different from being under the law?
Under grace, the Ten Commandments are not a criteria for salvation nor a legalistic system of priesthood, sacrifice, and law-keeping as Israel was under. Grace is as different from law as daylight is from dark, changing the believer's response to God. It is not permission to sin; that thought is to be banished.
Les explains that saying we are not under the law does not mean the Ten Commandments are forbidden or cast aside. They still establish a good society, and when societies rebel against those basic tenets, they get into trouble and empires fall. But the Ten Commandments are not a criteria for salvation, nor a set of doctrines, and believers are not under them as Israel was with its legalistic priesthood and sacrifice. Paul refers to that legalistic system, and we are not under it. Being under grace is totally different from Israel being under law, as different as daylight is from dark, in our response to God. Then verse 15 asks whether we should sin because we are not under law but under grace; the King James says God forbid, and Les prefers, banish the thought.