When I say someone is "Jewish" I mean to say he is a religious Jew. One can be Jewish by ethnicity, but I am not talking about this here. The question I want to ask is why someone is a (religious) Jew? There really are only two answers to this question if you ask people around. One answer is that the Jew was born into a Jewish family and he learned his religion so he followed it since then. Another answer is that one converted to Judaism because he found the religion very beautiful.
I am just going to guess at the numbers. My guess is that 98% of all Jews have been born Jews and that only 2% are converts to Judaism. Every Jew, or nearly every Jew, is a Jew because he is either following traditional beliefs or that he found the beliefs beautiful so he followed them. What is interesting is to ask the same question regarding science. There are conflicting ideas behind abiogenesis, it is still an unconfirmed science. Ask a biologist why he has one particular stance on abiogenesis. He will tell you a detailed explanation and justification for his own views. The same is also true, but to a much lesser extent, with politics that people have. Ask someone why he has particular views regarding an issue and he will give you an explanation for them.
Scientific views are based on an argumentation process. Political views, to a much lesser extent, are based on an argumentation process. That is, we ask questions, we try to find answers, we defend those answers, and so forth. This is true with many other views too. Ask me why I am a determinist and I will tell you an argument. I can tell you I am not an determinist because of tradition, I did not came from a family of determinists, I was not taught determinism as a kid, and so I do not continue any tradition. I am not a determinist because I thought determinism was beautiful but because I thought it was true (I happen to think it is elegant but that was not why I became one). My determinism is the result of argumentation, not tradition or appeal.
Judaism (and religion) is different. It is either tradition or else it is appeal. People like Judaism so they convert. Doing something because you think it is beautiful is fine to do with art, but that is not fine to do with truth. The truth does not need to be beautiful. People are Jews, for the overwhelming majority, because that is how they been taught as children. Not because of argumentation or reasoning but simply because they continue their tradition and are used to it. This does not make it true. As for the converts, who converted because they like Judaism, where is there argumentation? I never heard a convert say that he became a Jew because it was able to answer important questions and furthermore justify those answers. Instead, I hear converts talk about how they had questions about life, they found Judaism, it answered those questions, and they loved the answers, so they found meaning. Not that they heard the answers and saw the justifications behind them. That is never mentioned. Conversion happens because of appeal not because of argumentation.
Judaism and religion survive because of traditional and appeal. Not because of argumentation. When there is an entire belief system that is entirely based on tradition and what appeals to people rather than being the consequence of argumentation then we need to be extremely skeptical to what this belief system is. We need to be skeptical to its truth claims.