Deviation in the Islamic Aqidah And the Splitting of the Muslim Ummah
...Whilst the Companions (radiallaahu anhum) were alive and after they passed away (beginning of the 2nd century, after 100H), people of different backgrounds, beliefs and ideas entered into the religion, and they were from the offspring of the Persians, Jews, Christians, and idol-worshippers.
From these were people who entered the religion with good intentions, and a true desire for Islaam, so their Islaam was good. And amongst them were a people who entered into the religion with an evil intent, and their goal was to corrupt this religion. This faction of people began to spread their poison amongst the ranks of the Muslims from the time they entered into Islaam, and they had a big role in the great turmoil in the beliefs (aqaa'id) of the Muslims and in corrupting their minds, and entering evils and spreading innovations into the Islamic world...