How did witches supposedly get their powers?

They didn't know what they were doing, as they were paranoid superstitious Christians who conducted the infamous witch trials in Salem and elsewhere. They would administer such tests as "The Drowning Test," where they would take a suspected witch and try to drown her. It was a common belief that witches floated in water no matter what. So, if she was a witch, she would either float or she would somehow magickally escape and fly away. If she was not a witch, then she would, well, SHE WOULD DROWN, so the person was condemned to death either way.

They kept doing this to innocent people over, and over, and over again all because of the fear of the christian god.