There is an obvious reply to your comment: The Bible may be wrong about this (or any other) topic. But that does not mean that the Christian faith is inherently incorrect. It means that everything should be taken with a grain of salt, especially Bible verses wrenched from their context.
But leaving this issue aside, why do YOU think that Muhammad is NOT in heaven, other than the undeniable fact that he did not believe in Christ as being the Savior? Is God (the Christian God) so petty-minded that he will not grant salvation except by an arbitrary criterion? My Christianity is too robust and healthy to think along that line.
Muhammad honored Jesus as a great prophet. Why cannot we Christians return the compliment, and honor Muhammad (and Gautama, etc.) as legitimate emissaries from God? It is just simple courtesy, and in no way jeopardizes the unique status which we give to Jesus Christ the Savior.