Foundations of the Sunnah

Therefore, He didn’t leave anything good except that He made it clear to us; and He didn’t leave anything evil except that he made it clear to us. Thus, among the greatest causes for the Ummah’s salvation is clinging to the Aqeedah and Tawheed, its avoiding Shirk and bid’ah.

So the foundation of the causes, which saves (us) from fitan(i.e. plural for trial) is Tawheed, avoidance of Shirk, Bid’ah, and new matters in the religion.