After my father died, I started frequenting the SDSU campus Hillel. My regular shul was about a half-hour away, and I didn't have a car. Hey, I didn't even have my license!
One night, I found myself sitting next to a man with the most beautiful voice since Frank Sinatra (it should be noted here that I have a voice fetish. Builder actually has a decent set of pipes, which he shows off every Shabbos singing zemiros.) E was single, into theater--OK, he was a professional mime--and, as I found out within about two minutes of getting him to ask me out, Messianic. Or as I like to call it, "Christianity in a tallis." Since he was about as committed to his religion as I was to mine (I actually bothered to show up every week), it didn't work out. But beign with him got me thinking.
Once, at the San Diego JCC's annual Yom Ha'Atzmaut carnival, I was looking aaround and noticed a strange phenomenon. Various shuls had set up tables. Every stripe was represented, from Orthodox (various Chabads, Beth Jacob, and a few in La Jolla that were too far away to visit) all the way to San Diego's Reconstructionist and Renewal temples. There was even a table for the Humanist temple--talk about a contradiction in terms! (Humanism is basically Judaism minus G-d--in other words, what's the point?) As for the Messianic community, they had no table set up. Oh, they were there, all right--mostly picking fights with the Jews for Judaism guys. And I wondered, "you can be a Buddhist, an atheist, an agnostic, hey, even a pagan, and still call yourself Jewish. But you can't be a Christian Jew. Somehow that doesn't work."
Now, don't get me wrong. I once went with E to his I-hesitate-to-call-it-a-synagogue church, and the only thing differentiating it from a tent revival was the more-than-occasional yarmulke. So I had no illusions. However, I still needed a more consistent answer to my question about who was accepted and who wasn't. The only position that made sense was Orthodoxy. Basically, they're ALL illegitimate. And, since I would stop off for mincha every day after work as I got older, I was kind of halfway there anyway.
I think I am the only person who had missionary activity backfire like that. E tried so hard to sell me on Messianic. I even beat his I-hesitate-to-call-him-a-rabbi minister in a debate, point for point, Scripture for Scripture. And, after all that, I end up Orthodox. Probably the only person who became Orthodox BECAUSE of a Christian missionary. In fact, had I never met E, I'd probably be complacently Conservative today.