Is a mosque's promotion of an activity organized by a church during Christmas - with the promoters' knowledge of the activity's content - considered Kufr (disbelief)? And is it obligatory to establish proof against the promoters before ruling them as disbelievers? And what is the ruling on one who defends these promoters?

It is not permissible for a Muslim to cooperate with disbelievers in their festivals, such as guiding people to their churches or promoting their celebrations, because that entails cooperation in sin and falsehood, and participation in the establishment of idolatrous rituals that claim God has a son.

This act is a grave sin, even if it does not reach the level of disbelief that expels one from the faith, as long as the perpetrator does not validate their religion. Those who commit this reprehensible act must be advised to abandon it.