[rdb3]

Question for you. Why is it that a Jew (I'm speaking in the ethnic sense) who becomes a Christian is no longer a Jew, but a Jew who is an atheist remains one?

[Nachum]

It is a matter of Jewish Law. A Jew that makes a public declaration to attach himslef to another G-d is considered an apostate. It falls into the category of "idol worship". Idol worship is considered an extremely grave sin. While denial of G-d is considered a sin, it is not the additional step of attachment to another.