Being Jewish means being part of a community of shared values and beliefs. If you have rejected the overwhelming majority of the community and its beliefs, it isn't a stretch to say that you have departed from being a Jew. To put it another way, what will such a person leave behind in this world that could be identifiably be called Jewish. If the answer is nothing, then that is also what remains of them as Jews as well. Being part of a group always works both ways. You have to want to be a member and the group must accept you as such. Neither is really true of the people in question.
Being Jewish means being part of a community of shared values and beliefs. If you have rejected the overwhelming majority of the community and its beliefs, it isn't a stretch to say that you have departed from being a Jew. To put it another way, what will such a person leave behind in this world that could be identifiably be called Jewish. If the answer is nothing, then that is also what remains of them as Jews as well. Being part of a group always works both ways. You have to want to be a member and the group must accept you as such. Neither is really true of the people in question.