I'm no expert on what the main tenets of Judaism are, so I will accept your claim that this is one of the main tenets of Judaism.
But jstone78 made the point -- which I believe is correct -- that a "Jew" can be an atheist, and nevertheless be accepted as a Jew, whereas a "Jew" who accepts Christianity is not accepted as a Jew. Is it really true that belief in God is not a main tenet of Judaism in the same way that belief that the Messiah has not yet come is? If true, it does seem mighty peculiar.
Well, the first thing that you need to understand Jews, is that if there are 4 Jews in a room, you'll typically have at least 5 views on any given topic!
Seriously, there are different "types" of Jews, for lack of a better term. There are Orthodox, Conservative, and Reform. However, the are subdivisions of those as well, and it's rare that they can agree on anything! For instance, there are some Ultra-Orthodox who refuse to accept converts as Jews, unless they went through their conversion under the supervision of Orthodox Rabbis, which actually threatened support of Israel from Conservative and Reform Jews!
There IS a basic tennant that is a child is born to an intermarried couple, that the child is considered Jewish if the mother is Jewish, but not the father. The reverse is NOT true. The reason being that the mother is responsible for raising the child. Depending on who you talk to, that child may or may not be considered Jewish. Even if he was raised Christian, Moselem, Hindu, etc...
There are a lot of different view on what exactly makes a Jew, and luckily, there IS a congregation that will accept you, if you just look for it.
If you deny the existance of G-d, I doubt that you'd be accepted into an Orthodox or Conservative congregation, if you voiced those opinions. On the other hand, a Reform temple might accept one who voiced those views, although for the life of me, I can't understand why someone who denies the existance of G-d would go to services!
Mark
My wife's side of the family is Jewish, and my sister-in-law is a Messianic Jew married to an Evangelical, so I get to see a lot of the dynamics up close
Being "Jewish" is both a religion and an ethnicity. One can be an atheist, yet still have a strong attachment and identification with the ethnicly-Jewish community. Once you become a Christian of Jewish descent, you identify more with the Christian community.