Omya,
In Islam you have not only the major sects, like Sufi, Shiite and Sunni, you also have splinter groups within those sects. I know that, as in the reports from Iraq this week, some sects are fighting others for participating in the new Iraqi leadership. I don't want to discuss that, I can only assume it's for the same reasons that Islamic forces in some places are moving against Christians.. that is, on ideology and theology issues. (Though if I am wrong in that assumption please tell me.. I am truly interested in understanding why).
However, back to my original statement, there are major differences in how each sect think and operate based on their understanding of the Qur'an. I'm currently reading a history of the development of Islam in various parts of the world, including some groups such as the Nation of Islam (NOI) in the US. One of it's founders was one named Robert Poole, who became known as Elijah Muhammad. He believed that the founder of the NOI sect Wali D Fard, otherwise known as Wallace D Fard, Wali Farad and Fard Muhammad Ali, was in fact Allah incarnate who had appeared as the long awaited second Messiah.
Over the next few years, various leaders led the NOI in different directions until another leader came on the scene in 1978 (Louis Farrakan) who preached that Elijah Muhammad was infact Allah's final messenger and not Muhammad, as orthodox Islam preached. Interestingly, this same Farrakan came to Australia to Australian Aboriginies preaching that same message as to who Elijah Muhammad and Wali D Fard were meant to be according to NOI teaching.
Now, the followers of this sect would call themselves Muslim, yet they are not recognized by mainline orthodox Islam. I'd also suggest that they are considered blasphemous by mainstream Islam, though I haven't heard what the official response from Islam is.
Now.. if that is true, is this any different from people who call themselves Christians, who use the Bible to support their views, and who are not recognized or accepted by other Christians? Even as mainstream Islam wouldn't join together with this sect so also, true Christians wouldn't join together with any group that preached another Gospel than the one we have been sharing with you in this forum.