How do you define a religion?

I was thinking about the definition of a religion, and decided it has the following features:

1.  Oral or written traditions.

2.  Group activities and rituals.

3.  A professional priesthood.

4.  A behavior code.

5.  A theory of the creation and of the afterlife.

6.  A sense of mysticism.

And I define it so that something is a religion if it has at least five of the six features.  Something like Confucianism is close, but it has only four features.

That's one of those abstract things I sometimes think about.  I've been thinking recently about three-dimensional geometry, and Platonic and Archimedean solids. (They're the three-dimensional figures formed with regular polygons as their faces.  The most famous example is the cube.)