Allah be pleased with them—in grievous terms that need not be mentioned here, besides other things. Thus, the Muslims fought them at the command of Allah and His Messenger.

If, during the time of the Messenger of Allah, may Allah bless him and grant him peace, and his rightly guided caliphs, there were people who professed Islam yet departed from it despite their great devotion to worship—so much so that the Prophet, may Allah bless him and grant him peace, commanded that they be fought—then it follows that someone who professes Islam or adherence to the Sunnah in these times may likewise depart from Islam and the Sunnah. Someone may even claim to follow the Sunnah while not being one of its adherents, but having departed from it. This happens for various “reasons”:

One is the excess that Allah Most High condemned in His Book, saying:

“O People of the Book, do not exceed the bounds in your religion, and say nothing about Allah except the truth. The Messiah, Jesus, son of Mary, was only a messenger of Allah, and His word which He conveyed to Mary, and a spirit from Him,”

continuing to His words:

“And Allah is sufficient as a Guardian.”

Allah Most High also said:

“Say, ‘O People of the Book, do not exceed the bounds in your religion beyond the truth, and do not follow the desires of people who went astray before, led many astray, and strayed from the right path.’”

The Prophet, may Allah bless him and grant him peace, said:

“Beware of excess in religion, for it was excess in religion that destroyed those before you.”

This is an authentic hadith.

Another cause is division and disagreement, which Allah Most High mentioned in His mighty Book.

Another is reports narrated from the Prophet, may Allah bless him and grant him peace, which those knowledgeable in hadith unanimously agree are falsely attributed to him. A person ignorant of hadith hears them and believes them because they accord with his assumptions and desires.