Clearly, there is a religion called Judaism, a set of ideas about the world and the way we should live our lives that is called "Judaism." It is studied in Religious Studies courses and taught to Jewish children in Hebrew schools. See What do Jews Believe? for details. There is a lot of flexibility about certain aspects of those beliefs, and a lot of disagreement about specifics, but that flexibility is built into the organized system of belief that is Judaism.
However, many people who call themselves Jews do not believe in that religion at all! More than half of all Jews in Israel today call themselves "secular," and don't believe in G-d or any of the religious beliefs of Judaism. Half of all Jews in the United States don't belong to any synagogue. They may practice some of the rituals of Judaism and celebrate some of the holidays, but they don't think of these actions as religious activities.
The most traditional Jews and the most liberal Jews and everyone in between would agree that these secular people are still Jews, regardless of their disbelief. See Who is a Jew? Clearly, then, there is more to being Jewish than just a religion.
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