Is it permissible for a Muslim to go to a Christian church on their holiday, and is the one who does so still considered a Muslim?

It is not permissible for a Muslim to go to the church to share in the Christians' celebrations or congratulate them on them; because that involves participation in falsehood and imitation of them, and due to the saying of Umar ibn al-Khattab: "Do not enter upon the polytheists in their churches on the day of their festival, for wrath descends upon them." And whoever does that, if he does not agree with their disbelieving acts or statements, it does not expel him from Islam, even if he has committed a grave matter.