The simple support of my statement is that you hear all the time about people who consider themselves "secular Jews" but you never hear about "Secular catholics" or "Secular Buddhists" or at least *I* never hear about these.

I know secular Buddhists. I don't know if any of my Catholic acquaintances are agnostics, but I've read accounts written by individuals who claim to be agnostic Catholics, and I have no reason to doubt their self-identification.

(We are talking about self-identification, right? Because even the Reform Jewish movement lists in its central tenets, "Belief in God as defined in the Shema"; agnosticism doesn't exactly qualify.)

Because once you're agnostic, you're no longer a Catholic.

That's incorrect. If you're talking about self-identification, there are certainly agnostics who identify as Catholic, even if they don't feel they are "practicing the Catholic faith". In fact, the Catholic Church makes that very distinction! Once you're baptized as a Catholic, you are "a Catholic" for life.

There are no atheist Christians.

Also incorrect.

The one difference is that once you join another organized religion, you are no longer Jewish.

I'd quibble with you over that one, there are people who identify as Buddhist Jews or Jewish Unitarian Universalists, I have a friend who identifies as Jewish and Quaker. (As far as Jews for Jesus, they're not even making an argument that the Jewish and Christian religions are compatible, they define "Jewish" in terms of ethnicity or birthright. You clearly reject that definition (no amount of religious conversion will change one's ethnicity or parentage), and so do I.)