What is the religious ruling on sects that do not follow the Prophetic Sunnah?

Allah Almighty has made it obligatory upon the Ummah to follow the Sunnah of the Prophet, peace and blessings be upon him. Whoever obeys him has obeyed Allah, and whoever disobeys him has gone astray in manifest error. It is not permissible to adopt the opinion of anyone who contradicts the Sunnah of the Prophet, peace and blessings be upon him. The Mujtahid Imams have commanded to abandon their own opinions and follow the Sunnah of the Prophet, peace and blessings be upon him, if their opinions contradict it.