What about Jews who have converted to Atheism? They are legion. Are they apostate? Or is belief in God irrelevant to status as a Jew?
An atheist has not "converted" to anything, but rather has lost his belief. He stands a better chance of returning to Jewish belief than does someone who has actively embraced another faith. Someone who becomes agnostic or atheist, but who continues to be observant, and who doesn't speak out against Judaism, would probably be handled more gingerly than someone who actively speaks out against his former beliefs. The object is to get the person to turn back to God, not to unduly punish him.