King Abdul Azeez (rahimahullaah) said:

Do you know what destroys the religion and (is the cause of) most of the trials between the Muslims? This has not (happened) except due to differing between the Muslims and their lack of being in agreement (i.e. differing in Aqeedah and Manhaj). [Source: Haqeeqah Manhaj Al-Mamlakah Al-Arabiyyah As-Su’oodiyyah page 17]

See here about splitting of the Ummah

http://www.aqidah.com/creed/articles/oeotc-glimpses-into-the-splitting-of-the-muslim-ummah-part-1.cfm