A masjid is known to have advertised for free shelter and food which is held at a Christmas event at a church, such people from the community clearly know it is a church which the masjid has been advertising for, and yet say ‘what is wrong with that’ when asked regarding it. 

Has such an act reached to the level of kufr, since without a doubt every muslim who adheres to the methods of dawah (regardless of his ideology) is completely aware of the fact that Christians use these funded facilities as a way of giving their own dawah, which is contrary to the aqeedah and minhaj of a muslim. 

Is it compulsory on a muslim to establish the hujjah on such people, and those that defend them in their acts that they have commited an act of kufr which has taken them out of the fold of Islaam? Or is it not even compulsory for such a muslim to establish the hujjah, since these are matters related to ilm Ad-Durruriyyah.


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.
782,
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, as we mentioned in the answer to question no.
146678. 

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. 

See also the answer to question no.
69811

And Allah knows best.