Non-observant Jews are just as Jewish as the Chief Rabbis of Israel. My statement was a bit strongly worded, and Judaism has religious aspects, but saying "one is not born with a religion" as argument against brit mila makes no sense because one is in fact born with a heritage of thousands of years, with a family, with a people. It is an extremely important part of the cycle of life as a Jew, together with the bar mitzvah, marrying, etc. Even many non-observant or not-all-that-observant Jews think brit mila is of importance. The American tendency to view Judaism as a religion just like Christianity while we should really be seen as another ethnic group with its own tribal rules and traditions (which, indeed, come from G-d) annoys me to no end, although I realize it's mainly Jews who have contributed to this in their attempt to assimilate and get rid of anti-Jewish prejudice.
I thought the word Judaism referred specifically to the religion, as opposed to the ethnicity. My bad if I was mistaken.
Of course I'm aware that there are non-observant (and even nonbelieving) Jews. But surely you'll agree that there is a (for lack of a better way I know of phrasing it) a "Jewish religion" that can be compared and contrasted to others, right?