What does Paul mean by "witchcraft" in Galatians 5:20? I recently came across this:


  Gal 5:19 Now the works of the flesh are manifest, which are these; Adultery, fornication, uncleanness, lasciviousness, Gal 5:20 Idolatry, witchcraft, hatred, variance, emulations, wrath, strife, seditions, heresies, Gal 5:21 Envyings, murders, drunkenness, revellings, and such like: of the which I tell you before, as I have also told you in time past, that they which do such things shall not inherit the kingdom of God.


Would Christianity have me believe in [the existence of] witchcraft, too!?
It wouldn't have you believe in witchcraft it would just have you avoid it. 

Whether you believe in it or not is entirely separate from it existing, see this wiki about Ouija boards.

So you don't have to believe that Ouija boards work, I personally don't. However, even though I don't believe they work, I still don't associate with those types of things. 

Incidentally this is related to  How does one test the spirits and identify false preachers?. 

The power of satan is real, just see the story of Moses in Exodus:


  7:22 And the magicians of Egypt did so with their enchantments: and Pharaoh's heart was hardened,


Which is why we have tests to know the power of the Lord from the power of the devil.