The OP's statement about a lack of community in Christianity is complete and utter anti-Christian propaganda.
"is it permissible for a Muslim to visit a Christian if he gets sick, or to attend his funeral if he dies? Is there any sin on a Muslim who does these things, or not?
Praise be to Allaah the Lord of the Worlds. He should not attend his funeral, but visiting him when he is sick is fine, because this may serve an interest, namely opening his heart to Islam. But then if he dies as a kaafir, he will deserve Hell, hence the prayers should not be offered for him. And Allaah knows best."
Muslims should not attend the funeral of a Christian.
Muslims may visit the bedside of a Christian who has fallen ill - in order to take advantage of the illness and proselytize.
Ugh, this is so empty of faith, love, and charity.