Settle a disagreement for me please. My friend said that she is under the impression that a person born a Jew who either 1) converts to Christianity, 2) is a non-believer or Atheist, or 3) is a Messianic Jew, cannot be buried as a Jew. She thinks any one of the above would have to convert back into Judaism in order to be recognized as a Jew. She bases this largely on the fact that she heard (Lashon Hara) that a Rabbi refused to give a Jew who “converted” to Christianity a Jewish burial. My response is that G-d made us Jews and we are unable to change that and we are always Jewish and are always entitled to a Jewish burial and we should be buried as Jews.
Your friend is right that we don't mourn for such people as written in the Shlchan Aruch (Yoreh Deah Section, 345:5), and it is custom to bury them in a separate lot in the cemetery so that kosher jews would not have these people buried next to them. Of course, this does not regard non orthodox jews, but jews that converted to another religion and do not want to be called jews. Hence, I am not sure about an atheist if he is like a convert to christanity, or a messianic. Rabbi Ro'i Margalit