Once again I'd like to point you to Rachel's excellent blog, Velveteen Rabbi. Some time ago, I asked a question about modern Judaism, one that she promised to answer in a future post. She did so, and wow has she given me a lot to think about.
My question was essentially this: Since modern Judaism seems to be quite comfortable welcoming atheists, Buddhists, and all sorts of folks whose aim is to flavor their world view with a bit of Judaism sprinkled over something else, why is Judaism still so firmly opposed to accepting Jesus Christ?
Rachel admits that the modern definition of Judaism (which, remember, is broad in part because it is both a cultural and a religious descriptor) could be summarized as anyone who wants to identify with Judaism, so long as he/she doesn't believe in Jesus.
In a lengthy and excellent post that Rachel calls Wrangling with J4J, she sheds a lot of light on the thinking of many in the modern Jewish community and the distinctions they see between Christianity and Judaism. Rachel is the first to admit that she represents the more liberal wing of Judaismshe doesn't attempt to speak for everyone. But she makes many interesting points, and sheds much light on Jewish perceptions about the Messianic ministry known as Jews for Jesus.
Go read her post and the comments she has garnered. If you are interested in better understanding modern Judaism, let me suggest that you make Velveteen Rabbi a regular read.