There is no one experience I can point to and say "This is what led me to become Jewish." Jewish beliefs have always struck a chord with me, even when I was a small child and my parents were doing their level best to mold me into a model Catholic. Among other things, I like the idea of a faith where questions are not only permitted, they are encouraged. (I think I've already posted about how I got kicked out of CCD because I asked a question - a factual question, no less.
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I hasten to add that I am a Reform Jew. I object most strenuously to the Orthodox treatment of women.