One supposes a Christian mage would be a priest or a monk or nun.
There's a precedent of a magic-using Icelandic Christian! Sæmundur fróði Sigfússon, a priest and a scholar who reputedly went to the Dark University to study magic under the devil's tutelage. He should have lost his soul in bargain but managed to fool his teacher and escaped, and after that he spent his whole life fooling the devil again and again until he died peacefully of old age, the devil never getting his part of the deal.
There are numerous legends of Sæmundur, who actually was a real person and made such an impression on people that he turned into a bit of a legend. Despite him being a Christian priest there was never any attempt at hiding the fact that he used magic a lot, for various purposes, and that he had sold a deal on his soul to the devil on several accounts (or promised him that if he does thing X without failing he can finally get what was due to him, and then making the devil fail at it). He's also portrayed as rather mild-mannered, forgiving and gentle person... I remember only one story where he gets annoyed and that too only for a moment.
Sæmundur was literally a Bible-thumper; he heard there was a good position open in Iceland and made another deal with the devil so he'd bring him there fast enough so he'd be the first to apply for it. The catch was that he wouldn't get wet during the trip, which the devil accepted and started carrying him to Iceland on his back. Sæmundur spent the trip reading the Bible and when Iceland was close enough he smacked the devil on the head with said Bible, making him faint and sink into sea. Sæmundur himself had to swim for the shore but falling in water made the devil fail the deal details. There's a statue of this in front of the university of Iceland.
He was notorious for all kinds of mischief and didn't even blink if what he did was f.ex. against the law, and was always ready to help servant women who got in danger because the devil never went far away from him. He once saved a servant girl who had unwittingly promised her unborn baby to the devil, when the story makes it sort of obvious the baby was conceived out of wedlock, without even mentioning to her that she should be ashamed of either the baby or the deal she struck out of pure laziness. He also appreciated other magic users and was even outperformed by one elderly lady once when they were all making hay and had to hurry getting the dry hay indoors: she sent it from the field by magical means and he was supposed to move it in but even with all his knowledge he failed to keep up with her.