My friend Jenny Moyers is not the most connected Jew ever to walk the earth. She doesn’t belong to a synagogue and does not celebrate any of the Jewish holidays in her home. She seems to regard my rabbinic career path with something bordering between amusement and disdain.

So it came as something of a shock to me when one day we were walking together in the Philadelphia subway and a yellow-shirted Jews for Jesus missionary approached us to offer some literature. Jenny went completely ballistic and started yelling at him at the top of her voice, screaming that he ought to be ashamed of himself for calling himself a Jew. “Jews do not believe in Jesus!” she yelled. “What you are doing is disgraceful!” Needless to say, Jenny’s outburst made quite an impression on me, and it got me thinking.

Jews for Jesus push a lot of people’s buttons–even people who would otherwise not really care about Jewish tradition or practice–because they reside at the messy little intersection of identity and belief. For the most part, you can believe (or not believe) and do (or not do) whatever you want and still be Jewish. That’s because being Jewish isn’t a function of a particular belief or set of actions so much as it is a cultural, historic, and spiritual identity into which you are born or choose to convert. You’re just Jewish and, like Jenny, you don’t really need to worry about the particulars.

Jews for Jesus messes that all up.

If you really can believe whatever you want as a Jew, couldn’t you be Jewish and believe in Jesus? The answer is no, and I think it has less to do with theological objections–although these certainly exist–than it does with identity issues. For 2,000 years, at least in the West, Christians are what Jews defined themselves against. Oppressed, victimized, expelled, and slaughtered simply for who they were, Jews had their identity and outsider status reinforced over and over again. They were Other, and the oppressors were Christians.

The symbol of Christianity par excellence, the defining element, is Jesus. So to hear the words “Jew” and “Jesus” strung together into the phrase “Jews for Jesus” hits a very raw nerve for many Jews today–I imagine something akin to what Jews for Allah(!) would do to Jews who were oppressed in Islamic societies.

It’s not out of hatred of Jesus, or of Christians, or of Christianity, but rather as a reaction to hundreds of years of oppression. And for the many Jews like Jenny who don’t participate in Jewish life in any way, rejecting Jews for Jesus affirms their own bona fide Jewish identity. But, perhaps just as revealingly, the forceful reaction also acknowledges an underlying insecurity and doubt about whether you really can do or believe whatever you want and still be Jewish. Because the truth is, you can’t–there are some lines that just can’t be crossed.