Okay -- but in this case, the First Commandment does become an ethical obligation, as does the circumcision of your sons. The obligations arise from the convictions, not from the membership in the club.
That's just a chicken-egg problem: are people religious because they have convictions, or do they have convictions because they're religious? I'm not sure you can conclude that the former is true rather than the latter. In any event, just because a person may be convinced that god, e.g., doesn't want him to eat rabbit, that doesn't necessarily mean that the person is ethically obligated to refrain from eating rabbit. Even if god commanded everyone -- not just members of a particular religious sect -- to abstain from rabbit, that still doesn't make abstaining from rabbit an ethical obligation, since people would be acting out of fear of punishment rather than for the sake of what is right.
On the flip side of that, I'll concede that the obligations would not exist for an atheist Israeli who for opportunistic reasons goes along with the Jewish culture he lives in.
Then you're setting up a relativistic morality, aren't you?