Are the people of innovation considered Muslims, and what is the ruling on reviling them if they revile the Companions and the wives of the Prophet (peace and blessings of Allah be upon him)? Is it permissible to pray behind them or enter their places of worship?

In summary, innovations (Bid'ah) are of two types: an innovation that constitutes disbelief (Bid'ah Mukaffirah), such as the innovation of claiming the Qur'an is created, or denying Allah's attributes, or claiming that the Companions apostatized after the death of the Messenger (peace be upon him), or claiming that the Noble Qur'an has been distorted, or claiming that saints know what was and what will be, or slandering Abu Bakr and Aisha (may Allah be pleased with them) despite the Qur'an's commendation of them, or claiming that Allah Almighty indwells in any of His creations.

There is also an innovation that does not constitute disbelief (Bid'ah Ghayr Mukaffirah): such as the innovation of celebrating the Prophet's birthday (peace be upon him) as an Eid, and what people have introduced of reciting the Qur'an before the Fajr adhan, and adhering to specific dhikr with certain numbers at specific times in specific forms.

Whoever commits an innovation that constitutes disbelief, it is not permissible to pray behind them. As for those who commit innovations that do not constitute disbelief, it is permissible to pray behind them.

As for going to places where innovations and reprehensible acts occur, it is not permissible, lest the delusions and misguidance of the innovators be imposed upon those who are not armed with religious knowledge. Allah Almighty said: {And those who do not witness falsehood, and when they pass by ill speech, they pass by with dignity} [Al-Furqan: 72], meaning they do not attend forbidden speech and acts, thus avoiding all gatherings that contain forbidden speech and acts. Sitting with people who commit reprehensible acts is not permissible, and anything that is forbidden is also forbidden to watch, because it is a form of aiding in disobedience.

As for reviling them: it is not permissible if there is a fear that they will revile the Companions (may Allah be pleased with them), as a preventive measure. Allah Almighty said: {And do not insult those they invoke other than Allah, lest they insult Allah in enmity without knowledge} [Al-An'am: 108].