Is it permissible to participate with the Christians in their prayer of mourning for their dead?

Question:
May Allah reward you. The Honorable Shaykh, he says: A relative of mine abroad asked me to pray with the Christians in their prayer of mourning for their dead, so I responded to them out of politeness. The question is: does his action affect his faith?

Answer:
It is not permissible for him to participate in their polytheistic and disbelieving prayer; because after the sending of the Messenger (peace and blessings be upon him), there is no prayer except what the Messenger (peace and blessings be upon him) legislated. There is no prayer of the Jews, no prayer of the Christians, and no other prayer; there is no prayer except what the Messenger (peace and blessings be upon him) brought. So he has erred in this action and committed a sin; therefore, he must repent to Allah (Mighty and Majestic), seek His forgiveness, and not return to such a thing. Yes.