Eorl,
that's a very contemporary issue. In the earliest Jewish literature, the existence of God is taken for granted. There are no proofs offered, no defense of belief.
Orthodox Jews tend to reject the idea of atheist Judaism for pretty much the same reason you give: "what's the point? Why bother? How can someone belong to a religion that speaks of a convenantal relationship between God and the Jewish people and not believe in God?"
But outside of Orthodox Judaism views on this issue differ. One important difference is that a liberal Jew might not see any authority in the mitzvot or they might view the commandedness in a different way. I'm with Franz Rosenzweig on this who said it's about feeling commanded, in the way we feel commanded in any relationship, by relationship with God, awareness of God's presence, to act differently and instill our lives and actions with that awareness. The mitzvot are the way Jewish people do this. I'm with Franz on that for me but there are many other approaches. I actually don't limit myself to this single approach alone.
So there's humanist Judaism which, as the name seems to suggest, is atheist. Taken from their site:
Quote: Each Jew has the right to create a meaningful Jewish lifestyle free from supernatural authority and imposed tradition.
The goal of life is personal dignity and self-esteem.
The secular roots of Jewish life are as important as the religious ones.
The survival of the Jewish people needs a reconciliation between science, personal autonomy, and Jewish loyalty.
http://www.shj.org/believe.htm
There's also reconstructionism which, as I believe I mentioned, was founded by a man with a view of God entirely removed from the traditional one. A quote from him:
"God is the sum of all the animating organizing forces and relationships which are forever making a cosmos out of chaos." He rejected the supernatural. He also called the mitzvot folkways.
Reform Judaism rejects that anything but the moral law is binding and leaves it upon the individual to choose what ritual law to observe.
Myself as a liberal Jew, I don't feel commanded by God to follow the mitzvot. I don't believe God gave the Torah at sinai. I don't believe Torah is divinely inspired. I hold that Torah is only sacred because I consider it so. I find that the rituals allow me to increase my sense of awe, raise the mundane beyond the mundane. And some of the mitzvot do have an other specific goal attached to them, like Shabbat. I do go with Mordecai Kaplan on viewing the mitzvot as folkways.
Even for the Orthodox, it's not simply about being commanded by an authority but also the effect of following the mitzvot in such a way on one's experience of the world. When the authors of the new testament called the mitzvot a carnal law, they did a great disservice to the Jewish people.
Dauer