Wrong, the source of the power is their genes. Who decides your genetic structure? I was told it was God.
There's nothing derisive in the word "muggles", they're only made fun of by the bad guys, the good guys like muggles and consider protecting them to be part of their duties as people granted great gifts. Some wizards even think muggles are pretty ingenious in all the interesting inventions they make to get by without magic. The only muggles universally derided are Harry's relatives, because they're jerks.
The good wizards do consider it an honor and a priviledge and are humble about it. They don't consider themselves superior, as I said some even consider the muggles superior because of their clever solutions to living in a world without magic, the Weasley father is positively facinated by muggle inventions.
nobody is saying anything about a good power without God, that's your invention.
The Harry Potter books are all about children who were born with these powers and are learning to control them and use them for good while in the face of serious evil. They could just as easily be used for evil, that's part of what makes the bad guys so bad, and the good wizards take it as part of their duty not only to protect each other from the people using their powers for evil but also the muggles. Duty, honor and priviledge is a big theme in the books; what makes the bad guys in the books bad is that they abuse their priviledge and don't believe in duty and honor.
Then it is up to God what should be done with the powers. That's kind of THE WHOLE POINT.
Pagans believe and teach that we can have power granted by nature to do what we want to do. Sure, we should do only good things with these wonderful powers, but that's just because we care about people and the earth.
On the other hand, the Bible teaches that any powers you have that other people (muggles) don't have either comes from God, for His purposes only, or from Satan and the demons, even if you say you're using the power for "good".
People who don't attribute their powers to God and don't use their powers for his purposes (i.e. foretellers) are strongly distinguished from those who have special abilities from God, for God's purposes (i.e. prophets).
The belief that you can have supernatural powers to do what you want to do (whether or not you only do what you personally think is right) is exactly what the Bible condemns when it speaks of witchcraft.