Some people claim that a Muslim should not call himself other than Muslim. They quote the verses of Quran which denounces sectarianism, and as such, they conclude that being a member of any party is not allowed for Muslims.
It is true that Islam is against sectarianism and dividing into sects. However, being a member of a party does not necessarily mean sectarianism unless such party is a sect itself.
The opinion that one should not call himself other than Muslim, contradicts Quran. As a matter of fact, Allah sometimes has used other terms than Muslim, when referring to a subset of Muslims. For instance, in a couple of places in Quran, Allah mentions a group of Muslims with the name "Hizbullah" which means "The Party of Allah". If being a member of ANY party is resented in Quran and one should call himself Muslim and Muslim only, then Allah will become sectarian (may Allah protect us) by promoting His own party! The fact is that, Allah uses a different name because He wants to address a highly virtuous subset of Muslims. In fact, any member of "The Party of Allah" is a Muslim, but the reverse is not necessarily true. Some Muslims are weak Muslims, and some are just Muslim in ID, and as such, these people do not belong to the Part of Allah about whom Allah said:
"Verily the Party of Allah are the real prosperous." (Quran 58:22)
This goes to prove that not any party within Islam are condemned. In fact, the origin of the word Muslim goes back to Prophet Abraham (AS). Quran states that Prophet Abraham (AS) was a Muslim:
Abraham was not a Jew, nor yet a Christian; but he was an upright man and MUSLIM, and he was not of the idolaters. (Quran 3:67)
Also in another verse Allah states that Prophet Abraham is the one who has named us Muslims:
It is the religion of your father Abraham who has called you MUSLIM from before and in this (revelation). (Quran 22:78)
In another verse, Prophet Abraham (AS) advises his sons not to die without being Muslim:
And this was the advise that Abraham left to his sons and so did Jacob; "O my sons! Allah has chosen the faith for you; then die not except in the faith of Islam." (Quran 2:132)
Now, surprisingly enough, Quran testifies that the Prophet Abraham was a Shia (follower; a member of the party) of the Prophet Noah (AS):
"And most surely Abraham was among the Shia (followers) of him (i.e., Noah)" (Quran 37:83)
One may ask, why Prophet Abraham (AS) who has been called Muslim and also advised others to be Muslim till death, has been named Shia? This leaves no room but to believe that his being the Shia of Noah (AS) does not contradict his being Muslim.
Now, we realize that being a member of a party does not contradict our identity as being Muslim AS LONG AS the leader of the Party is assigned by Allah, or at least, as long as the leader does not order something against the order of Allah and His Prophet.
Suppose there exists a party with a leader whose name is Imam xxxx. One may subscribe to this party as long as he does not prefer the order of Imam xxxx over the order of the Prophet (PBUH&HF).
When does a party become a sect and thereby denounced by Allah? The answer is that it will be a sect if Imam xxxx states something against the commandments of Allah or His Prophet, and when we as followers prefer the order of Imam xxxx over the order of Allah and his Prophet (PBUH&HF). This has been severely condemned in Quran, and such party is no longer a school of though within Islam but it rather has divided his followers from the religion of God and has split into a sect. May Allah protect us from such parties.
source : http://www.al-islam.org
