I wouldn't say that christianity(or any other relegion for that matter) is not anti-selfishness.
People don't want to accept that after death they will be completely gone and are scared by that. thought
So they flee to religion which tells them that whatever they do their 'soul' will live on forever(may it be in heaven or hell). And since god is all-forgiving it really doesn't matter if you strictly follow his rules or not, you will land in heaven.

So I would say believing in a god is really selfish.

Even if you strictly follow the rules you are in fact selfish, because your only motive is getting a pleasureful afterlife for yourself and not actually helping others.
It is not different at all from doing it for money or reputation.

By the way there is no such thing as selfless behaviour. Everything that seems selfless is actually selfish.
People help other people not for nothing, but for one of the following two simple reasons(Actually these two reasons are responsible for everything you do):

• An expected increase in positive feelings(dopamine or other hormones released in their body)
• An expected decrease in negative feelings(other hormones like stress hormones not being released anymore)

Then a person would need to explain the odd phenomanon of people that are ready to die for a crazy man.

I can't explain the phonemon, but I can give you another example of it: Adolf Hitler.