How do you become a witch?
Well, some of us are born to the tribe. They say that He Himself overshadows our fathers at the moment of our begetting. That explains why we turn out the way we do.
But like other tribes, the Tribe of Witches tends to be porous around the edges. Opting in is always a possibility. (So is opting out, but that's another matter. And you know what they say: Once a witch, always a witch.)
You can marry in. Love is the ultimate bind-oath. Once you speak the language, eat the food, and keep the holidays, you're more or less in by osmosis.
Or you can adopt in. The old rites of initiation are essentially rites of adoption: you make the bind-oath to the gods of the thede (tribe), you're blooded, and you're in.
Because (with all due deference to Uncle Gerald) we're not really a religion at all.