Shaykh Fat-hu Al-Qadasee said, “The ruling of giving classes and lectures in the mosques of the common folk (i.e the laymen who are not callers to or teachers of their innovation) from Ahlu Bidah varies in view of the following:
Firstly: There must not be a Masjid belonging to Ahlus Sunnah in that locality, and one resorts to their mosque (only) if he cannot call the people (that go to that Ahlu Bidah mosque to the truth) nor caution them (from evil) except through that (i.e. attending their mosque).
Secondly: The level of fanaticism shown by the common folk (who go to that Ahlu Bidah mosque and are not teachers/callers of that bidah) to Ahlu Bidah (i.e. there level of fanaticism to the misguided sect that the mosque they go to is aligned with).
Some of the common folk (who go to Ahlu Bidah mosques) have love for the truth even though they are classified members of a particular (misguided) organization or sect, since they do not have a clear picture of the evil enclosed by these (misguided) organisations (or deviated sects). Their sheer love for the truth and lack of discernment has drawn them into allowing such people to give talks and lectures in their mosque (as they think these people are calling to the truth).
Thus there is nothing wrong with visiting the likes of these mosques (as they contain and are run by common folk who dont teach or call to their bidah and only allow callers to their bidah to give lecturers in their mosque because they truly believe these callers are calling to the truth). (When the student of knowledge visits he) gradually advises them.
Thirdly: As for those (common folk) who are (closely) associated with Ahlu Bidah inwardly (in aqeedah) and outwardly (in actions and speech), bearing allegiance and disavowal for their sake, being disinclined towards the sunnah (the truth) and its people, then one must not give lectures in their mosques.
This approach will be of no avail and may perhaps lead to him (the student of knowledge) disputing and being in confrontations with the hizbiyyoon (partisans). (This) wastes his time and harms his dawah (to the correct aqeedah and manhaj) as a result.
This is what I (Shaykh Fat-hu Al-Qadasee) see to be correct in this regard and Allah knows best.”
Posted by Abdul Kareem Ibn Ozzie