Holding on to the Rope of Allaah entails following the teachings and example of Prophet Muhammad, peace be upon him, in actuality. The Prophet, peace be upon him, taught us to protect our religion from those who would enter it and vainly try to corrupt it. The Muslims who defend the religion of Islam from the misinterpretations of the ignorant are trying their best to have the Muslims unite upon truth... not falsehood. So that the Muslims of the present and future will worship Allaah correctly upon clarity and not confusion.
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"Regarding the Shee'ah & the Correct Islamic Position Towards Them"
http://abulhaarith.wordpress.com/category/the-raafidah-sheeah/
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The reason why the Shi'a are not accepted nor reconciled with is because they have introduced beliefs and practices into Islam that were never taught by the Prophet, peace be upon him, never taught by his family, or his companions. Rather they have changed Islam from it's original perfected form and thusly, innovated a way of life that leads to Hellfire. This is not a dispute between a trivial "this" and "that", it is a serious issue dealing with the difference between the Straight Path to Paradise and the crooked paths that lead to punishment in this life and the next. May Allaah rectify our condition and affairs.
With regards to Shee'ism, then it is not the Islam that was taught by Prophet Muhammad, peace be upon him. Rather, they are a group that have exaggerated the status of the noble family of the Prophet, peace be upon him, to the extent that they build shrines devoted to their worship along with Allaah. This is shirk, polytheism, the greatest crime a human being can commit in the sight of Allaah.
The proper way to practice Islam is to follow the understanding of the Qur'aan and the Prophetic life example that was actually taught by Prophet Muhammad, peace be upon him, and actually lived by the Prophet, peace be upon him, his family, and his companions.
All beliefs and acts of worship that oppose this way are affairs that lead away from Paradise and lead towards Hellfire.
It is not permissible to worship Allaah with acts of worship that did not exist in the time of the Prophet, peace be upon him (such as pilgrimage to Karbala, calling upon 'Ali or other human beings for help who are dead or absent, building shrines dedicated to saints, etc...) And it is not permissible to interpret the Qur'aan with meanings that oppose what the Prophet, peace be upon him, actually taught.
Remember, the main goal of every Muslim is to enter Paradise and to avoid torment in the Hellfire. The main goal is not unity in this life for the sake of "unity". If the Shee'ah have chosen for themselves a path to destruction, then it is not in any Muslim's interest to "unite" with them. Rather the sincere Muslim should advise them to return to the right guidance that Allaah created us and them for, and if they refuse, then other Muslims should be warned against falling into their severe errors. By this way, with Allaah's permission, we preserve the correct practice of Islam for ourselves and for future generations who seek to worship Allaah correctly.
Whether the Shia' as a whole will repent and return to the Straight Path...? Allaah knows best. This splitting amongst the Muslims is a test from Allaah to see who will sincerely and correctly follow His religion. He guides whom He wills and He leaves to stray whom He wills and Allaah is All-Knowing, All-Wise.
ADDITIONAL NOTE: "Fitnah" Makers?
It is oppression to call a Muslim who clarifies the difference between true Islam and deviant ideologies a "fitnah" maker. The people responsible for making "fitnah" are those who alter the pure religion of Allaah and turn it to something that leads our brothers and sisters to Hell.
If I knew someone who was in danger of being murdered, would I be considered a "fitnah" maker if I were to alert that person to danger? No. I would not be considered a fitnah maker, even if the murderer was a Muslim.
And innovation in religion and the threat of Hellfire is more serious a danger to a person than murder and thusly a Muslim has more right to be warned against it in the clearest terms possible.