What is the ruling on some Muslims advertising a Christmas event that will be held by a church?

Praise be to Allah.

In the answer to question no. 160470 we stated that it is not permissible to take a Christian to a church, because that comes under the heading of cooperating in sin; rather it is the worst of sins, because of what will be heard in the church of blatant shirk (association of others with Allah), such as their claim that Allah, may He be exalted, has a son! There is no difference in the ruling between this and showing a Christian the way to a church on the occasion of one of their religious festivals, and promoting and advertising that. In fact this is an even greater sin, because it involves cooperating in holding those religious festivals that are based on falsehood. So this action of theirs is helping in two grievous sins: the sin of going to the church and the sin of participating in one of their religious festivals. In the answers to questions no. 69558 and 50074, we quoted from a number of scholars that it is haraam to help the Christians in establishing and celebrating their festivals. Undoubtedly the activities that the church will hold on the occasion of Christmas is obviously a celebration of that festival. 

Based on that, those who promote the religious festivals of the Christians and tell people about where they are being held are in grave danger, and undoubtedly by doing this they are falling into a major sin.

The scholars of the Standing Committee said: It is not permissible for a Muslim to cooperate with the disbelievers in any way whatsoever with regard to their festivals – which includes announcing their festivals – or promoting them in any way, whether that is via the media  or… 

Shaykh ‘Abd al-‘Azeez Aal ash-Shaykh, Shaykh ‘Abdullah Ghadyaan, Shaykh Saalih al-Fawzan, Shaykh Bakr Abu Zayd

 End quote from Fataawa al-Lajnah ad-Daa’imah, 26/409 

Is the celebration of Christmas by the Christians – originally – the celebration of the birth of ‘Eesa (peace be upon him), the human Prophet? The answer is: No; rather it is a celebration of ‘Eesa as “God” or the “son of God”! Exalted be Allah far above what they say. So how can a Muslim believe that this is participation in celebrating the birth of a Prophet, when in their view he is “God” or the “son of God”? 

However, participating in or helping with such events does not necessarily constitute kufr that puts one beyond the pale of Islam, so long as the one who does that does not regard their religion as being true. Hence we do not think that merely by doing that they fall into major kufr that puts them beyond the pale of Islam. Therefore we think that it is sufficient for you to denounce them, advise them and exhort them to give up what they are doing. There is no need to focus on the ruling on their action and whether it is kufr or not. What matters is that you should know that their action is haraam, and you should strive to make them stop what they are doing. 

And Allah knows best.