It has been discussed many times on this site that a non-Jew may not observe Shabbat (in addition to various other mitzvot that are prohibited to non-Jews). We have also seen that a Jew who renounces Judaism, while not losing his official Jewish status, may have to undergo some kind of process to reaffirm his commitment to Judaism.
My question is, may a Jew who has renounced Judaism continue to observe Shabbat? Must he? Does it matter if he has converted to some other religion such as Christianity?