Islam does not prevent you from role-playing characters of different religions. When you play single player, CJ is clearly not a muslim and does a lot of haram things. If you play any RPG games or fantasy games, your character may be of different religion or fantasy religion.
The church of sauron is only roughly based on cristianity - mostly the bad parts as with the GTA style (peadophilia jokes galore, corruptness and fanaticalism).
But actually you are free to operate the Church of Sauron as you see fit. Does not have to be based on christianity. Could make it more interesting to include some more Islamic traditions.
:pop:
Father Bastard
Cardinal of Los Santos,
San Andreas