Magic in a Christian Campaign
 
Can a Christian Player Character be a Magic User?
Yes.  During the first thousand + several hundred years of Christian tradition the separation of 'magic' and science simply did not exist as we know it today.  Many subjects condemnd by the ignorant today as 'Satanism' were taught in Christian Universities.  There is really nothing in the Christian Tradition to prevent a Christian from being a Magic User in an RPG.
Why would a Christian Study magic?
To glorify God.  The magical, occult, or spiritual reality is as much a part of God's creation as anything else.  In order to praise God in full, it must be understood and God praised for his handiwork there.  Thus God may call Christians to study magic.
To Fight Evil.  In order to fight evil demons, they must be known.  Thus there would be a need for Christians to know and understand these things.
To do good.  Not all of magic is evil.  It is governed by natural laws like the rest of the universe.  It is to be used to accomplish good.
Is D&D real magic?
No.  D&D was created by wargamers.  The 'magic' spells in D&D have no relation to any real world occult tradition, but are simply designed to allow players to accomplish tasks that are accomplished in our world by technology.  There is less relationship between the D&D magic system and real world magic than in many other Game systems.  So, for Christians who do not want to dabble in real magic, the D&D magic system is an ideal makebelieve magic system.