Such words are just an attempt to express in English what are essentially Arabic words. We find the same thing with the name of Muhammad which always used to be spelt Mohammed. The names mean exactly the same.
I suppose we see something similar with the British names Ian and Iain. And think of the multitude of spelling variants on the female name, Jackie!
Well, why would you see it spelt 'kristianity'? There is no translation between languages involved there!
The point I was trying to make - obviously unsuccessfully - in my earlier reply was that we are transposing Arabic sounds into English ones...or as near as different spelling/sound systems will allow.
And that's the problem...such a transference is not necessarily an exact science. What I might hear as 'Mo' is what you might hear as 'Mu' and what I might hear as 'e' is what you might hear as 'a'. That is why we get both Mohammed and Muhammad.
The spelling of that name in Arabic remains constant. Exactly the same applies to Muslim and Moslem and all similar 'translations'.
Moslem, Moslim, Mussulman and Mooslim have all been used to render the word into English over the centuries. You're quite right, Tom, in saying that Muslim is the commonest spelling now, but it was not always so.
That fact just supports what I said in my earlier responses...however we spell it, it is simply an attempt to render into English the sounds of a word from a foreign language and these may vary from time to time and between one person and another.
The word Mohammedan was also used before Muslim came to be widely accepted. I suppose this was the nearest equivalent to Christ-ian although could easily be Jesus-ian!
I wonder are all interpretations in the dictionary. Can I count on an Answerbanker to fill me in or will I, horror of horrors, actually have to go and look it up myself.
Mahometan, Muhammadan, Mahumedan, Mohammedan, Mahomedan, Mahommedan, Mohummadan, Mohammadan and Moohummudan have all been used at different times and by different English writers.
If any evidence were still needed that names/words/sounds from different languages are often transposed differently, the above list should clinch it!