Mohammedanism, Mohammedans, etc
In legal jargon, this is a virtually open and shut case. Even as the first edition of the LC came off the press, James Hastings wrote in his Encyclopedia of Religion and Ethics that "Islam is the name peculiar to the religion founded by Muhammad, and embraces all the different sects which are now found among his followers. This, a Shi'ite and a Sunnite are both Muslims." The proper terminology, then, for the religion and its professors is "Islam" and "Muslims." respectively.
"Muhammadam" according to Webster, is a term "used predominately by those outside the faith of Islam and usually taken to be offensive to the Islamic Believer."
Remedy: Islam, Muslims
Sanford Berman, Prejudices and Antipathies, 1971.