Before we look at what being a witch is today we must first look at what it was.....
What we now know as magick has been around since some of the earliest development of man came to be. Wiccan magick has been found to date back approximately 2500 years ago - centuries before Christianity came to be - examples of this early magick have been found in old cave drawings. This cave art depicts some of the earliest forms of sympathetic magick (a basic form of magick still commonly used today). Sympathetic magick is a ritual where you would use an object of likeness to what ur purpose was to demonstrate the real thing. For example native american tribes use this form of ritual before a hunt. One will dress in the pelt of a buffalo and and they will all chant and dance until the buffalo "dies" symbolizing the death of a real buffalo in the hunt.
Now fast forward a little to very early England before Christianity was established. The was a group established called the Wicca, or the wise ones. And after some time the king would never think to act on anything important without consulting the Witan - the council of the wise.
Fast forward a little bit more to the coming of Christianity. Pope Gregory the great decided to have the new Christian churches built on the grounds of the former wiccan temples he had the idols smashed and and holy water sprinkled around to rededicate the area to the "new god".
Fast forward just a little more and we find ourselves at the times of the witch hunts. The new Christians believed that pagans where devil worshipers because the appearance God - the horned one - matched the description of that of the Christian devil.