Esselean
What about those who are Christian witches? Surely they do not worship this 'supreme witch mother and witch father'
The thing about witchcraft is that it is a practice. You do not need a specific god or goddess to practice it.
Let's take an example of a simple spell: Cord magic. Nine knots in a piece of (usually) black cord. Each knot is supposed to hold the powers of the witch and the will of the witch.
Meet Maria, a nice Christian school-girl that practices witchcraft. Also meet Sabrina, a nice girl that doesn't worship anyone but practices witchcraft.
Now, both Maria and Sabrina want a date for the prom (yay prom) so they do a simple cord spell to "pull" in the date of their dreams. So, Maria invokes the power of YHVH the almighty and begins construction on her pink Cord.
Sabrina does the same except that she invokes within her personal power and triggers this into the cord as the chants her spell. She may even put a bit of saliva or blood on each knot to power it.
Voila, both get their dates.