Today is Rosh Hashana, went to services last night, as usual am suffering the effects of being half-in and half-out of this religion thing.
Religion is how people and communities establish a relationship to the transcendent, to the eternal, to the infinite, to the absolute, to the sacred, to things unseen and to the powers that underlie the world. So in some sense, you have to have a religion, even if it's one that denies that these things have any sort of reality or meaning whatsoever -- that too establishes a relationship. We all have to live in the world, we all have to deal with its immensity and our smallness.
If the above is how we relate to the cosmos, the other part of religion is about how we deal with each other. It is less clear to me that these things have to be managed by the same institution, but that seems to how things have evolved.
Judaism is very proud of the fact that it invented monotheism (highly disputed, Freud thought they got it from the Egyptians, here's an interesting looking paper that traces it back to Assyria), supposedly the best idea evar. Opinions differ, some say it's the worst. My own feelings (good for today only):
a) it's an important step in the evolution of the human mind, that is, it has approximately nothing to do with whatever is powering and governing the universe and a lot about how we construct and construe ourselves;
b) while it's a crucial part of the growth of western civ, including the devlopment of science, and thus is baked into the deep structure of my own mind, we are in a cultural point where we have to move on to the next thing. God is dead, but gods have a way of coming back from death, generally transformed in some way.
The flavor of Judaism I am currently involved with is the San Francisco fuzzy kind, so the kind of monism on display tends towards the mystical rather than the authoritarian. That's a lot more acceptable, although sometimes it gets too gloppy for me. Everyone's too nice, it leaves out the part of Jewish culture that resonates most with me, argument. On the other hand, insofar as it works for me at all, it works because the genuine spirituality of the community is capable of sneaking past my rationalist defenses.