In your case a "white" magician is actually channeling power from God- this would require the best behavior, faith, and caution. Protection spells would be especially strong, as would healing and the like.
Black magic (Satan) would be effective, but to make certain black magicians are in his grasp any such cleric would be required to do some pretty nasty stuff to get that power; causing disease would require making (good) victims suffer during the ritual, for example.
Demons and angels might come into it. If a black magician calls upon a demon a good cleric can call for help from an angel.
A paladin could have a sword that literally channels divine power in it, thus making it especially effective against demonic foes, or anyone using evil power. By the same token an evil warrior could have a weapon that channels evil power, causing pain, disease, whatever against a foe.
Nope, my totally not-cleric isn't a spell caster, he prays to God and is able to perform miracles this way. I'm calling it a not-Cleric because of the overhaul I'm giving it. I'm making it Christian but also removing some limitations that given my setting make no sense.
What I'm after is a White Witch, in my head Witches are born, some mutation or something in their lineage allows them to feel and manipulate magic energy. Women are more likelly to be a witch even if the Father was a witch himself. Women are also usually way more powerful than men.
Now a witch isn't automatically evil (IF I can manage to do this), but black magic is very tempting due to it's power and extended range of spells, almost zero limitations on what it can do (no spell can give life, only black magic spells can create a mockery of it).
Black witches "eat" white ones to feed their own powers.
There's firearms, modern ones, and no limitation on either The Blessed or a White Witch to use them or to draw blood if done in self defense or to protect the world from evil. (I mean kill with daggers, swords, etc.)
I'm almost done with The Blessed (My totally not-cleric)
The Witch is done but for the spell list.
If I can't find a cohesive and setting coherent division of spells and magic rules (I mean the rules the PC has to obey not the game mechanics) then I'll jetison The Witch and make do with The Scientist until such a time when I manage to make the class work inworld.