There are all sorts of answers to the question of “what makes one a witch?” The question here was in part inspired by a posting in Yahoo Questions some time ago, and there are several responses there as well.
It’s also been asked frequently elsewhere, such as The Witches’ Voice: Witches – Are they Born or Made?, and Born or Made at Traditional Witch Community Forum and many other places all over the ‘net, often quoting a bit that quotes Joe Wilson quoting Robert Cochrane “A Crafter is born not made, or if one is to be made, then tears are spilt before the Moon can be Drawn.”
For now, here’s one answer from the past, which also happens to be a quote from Robert Cochrane, which kind of parallels one by Ernest Thompson Seton as well:
“If a person is of serious intent, one night they can go out at midnight of the full moon and make their own pledge to the Goddess and the Old Gods in truth, honesty and sincerity.
This sort of pledge has as much validity as any formal oath sworn to a group.” ~ Robert Cochrane (1/26/1931 – 7/3/1966)
Of course, that won’t necessarily gain you the Keys to the Craft, either.