No. Once a Jew, always a Jew. Even if a Jew does not observe all or part of Jewish teachings and even if he denounces his religion and lives a life contrary to Judaism — he never ceases to be Jewish. In the words of Maimonides in a renowned letter he wrote to Jews in Arab lands who were being forced to leave Judaism for their religion: “It is impossible to escape. No matter what steps a Jew takes to forsake Judaism, he is still held responsible for any transgression and will also receive great reward for every mitzvah.
