Counterfeit money is lacking in authority—it does not have the backing of the federal government. A good counterfeiter can dupe some people into accepting his copywork as legitimate currency. But eventually, when the fake money is brought before the authorities, it will be found false and sentenced to destruction.
Likewise, a false gospel lacks something very important: the authority of Christ. The preacher of a false gospel may believe it and persuade (others) to accept its message. Nevertheless, in the final analysis, there is no salvation in this fake gospel.
Source: Green, M. P. (1989). Illustrations for Biblical Preaching Grand Rapids: Baker Book House.) Contributed by: Matthew Kratz