Defining Religion for the First Amendment

This quarter I'm enrolled in Freedom of Expression, which focuses on all aspects of the 1st Amendment. It's a really interesting part of the Bill of Rights, both in its historical intricacies and how the law coming out of the First Amendent looks. In the interest of posterity, here is the language in question:

Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the government for a redress of grievances.

And with those 45 little words comes American's basic right of expression, thought, and belief. Today's class, while a little on the pedantic side, was interesting thanks to a thoughtful homework assignment: What is "religion"? I limited myself to religion in the First Amendment sense, but the question remains largely the same. Suffice to say, the question is not easy to answer.

Most of class was spent broadening the definition first offered by students... does a religion really need to be organized, must the beliefs be shared, what is a belief? Which were all very interesting questions and lead to an increasingly broader definition. I took a very different tact when trying to answer the question: come up with a definition that includes an accepted religion group like the Catholic Church but excludes an accepted political group like NARAL. Turns out this is not such an easy task.

I eventually developed the following answer.

Religion for purposes of the First Amendment refers to beliefs which cannot necessarily be explained through rational thinking, are held by a large group, and deal with certain non-secular issues. Religious exercise must be confined to those exercises which do not infringe upon the rights of others.

I like the answer, but it really doesn't exclude NARAL as much as I'd hoped. I did get a good definition of non-secular from class: "spirituality and/or creation not grounded in observable fact." But the answer still seems to broad to be workable.

Clearly this class is going to give me a few things to think about.