Shaykh Saalih al-Fawzaan (may Allaah preserve him) said:
“…So nothing cannot unite the people except the sound creed. But if they should differ in their creed, they will never be united forever.
Source: A Gift to the Reader in Annotation of Sharh As-Sunnah, vol. 1, p. 22 by Shaykh Saalih al-Fawzaan (may Allaah preserve him).