I started a slightly similar story about five years ago (and abandoned it). So I have been down somewhat similar terrain. So I do have some advice.
Before I give my advice, I wanna say your idea is far better than mine. And it has potential for being downright profound. One of the fundamental characteristics about the entire Judeo-Christian segment of human history is that it's a brand of religion driven primarily by a written book as opposed to an erupting volcano or a pretty amulet or the imbibement of drugs. This fact is so simple that it's sadly quite easy for us Christians to miss how truly profound it ultimately is. It was a BOOK that changed the world. WRITTEN WORDS. It started with two stone tablets chiseled by the finger of God. And then the writings of Moses. And then the Torah. And then the New Testament. Everything points back to written words. Words are everything, and as a writer, words are the materials (bricks and mortar) you will use to build a story.
As for my advice on your story, I wanna give you an example from "Tarzan" and then I'll make my point from there.
The original story of "Tarzan" as written by Burroughs had ridiculous plot holes in that you should be mindful of while trying to write your tale. One plot hole was Tarzan's ability to speak English. He was raised by apes, and never saw a human being other than his parents who were killed when he was just an infant. So he never even heard a human voice, and certainly never heard English spoken (except as a tiny tiny baby). But he later found some English story books in the scraps from his former human life (the wreckage of his parents' airplane I believe) and by teaching himself to read these story books he taught himself HOW TO SPEAK English. And so when Jane and the others found him as an adult, he spoke English.
Hogwash.
While it might be remotely possible that he could have somehow--through some amazing gift of linguistic genius--learned to READ (in total silence) those English books, there's no way he would have learned to PRONOUCE them without someone constantly speaking actual English to him for years at a time.
Here's my advice:
If this wizard character of yours has never HEARD people discuss Christianity, then it would stand to reason that he--like Tarzan-would be getting all his information exclusively from this book (the Bible) and I assuming it's only going to be a King James translation, or else a Geneva translation (no New American Standard, and certainly no New International Version). So there's no way he could/should speak in contemporary strains of "Christianese." I have no doubt that via the Holy Spirit he will get a grasp of some rather profound spiritual concepts. But outside of the social context of contemporary Western Christianity he will in no way ever ever ever be able to know the following:
1) The Trinity - Not once does that 7-letter word appear anywhere in the Bible. The concept is Biblically valid, but he can't possibly ever hear/read that word anywhere in this world you are crafting. So he should never speak it (unless he gets his hands on ANOTHER document that mentions the Trinity--the Satanic Trinity, perhaps??-- and then he has a Eureka! moment where he pieces it together himself).
2) Vicarious - Also not in the Bible. Although, admittedly, the word DOES exist in the English language outside the context of the Christian religion. So it might be possible (again, via the Holy Spirit) for him to make the association between that word and Christ's sacrifice.
Maybe you can make your wizard a high-ranking scholar in his world with an acute adeptness at words. Maybe that's the hook he needs to be able to decipher what's happening in the words of this Bible thing he found.
Now that I've pointed to the words that are NOT in the Bible, how about the ones that ARE.
The word "Warfare" is found in the Bible, both OT and NT. In OT it's always only literal human warfare. In NT it's always only the spiritual kind.
The word "Atonement" is significant. In the OT it refers to the physical lid on top of the gold box that was the Ark of the Covenant. But in the NT it refers to the profound concept of erasing our sins. This might start off as something of particular interest to your character when he first starts with the Bible because the Ark of the Covenant is an obviously ritual-focused thing for him to be reading about, involving blood and incense. But then he will find that the rituals in the Bible are not centered upon spells and incantations to secure harm against and control of other people. Instead those rituals are appeals for the forgiveness of sins. Does the word "sin" even exist in your wizard's world?
The word "Covenant" is also a real kicker.
What's more, on top of the gold atonement cover sit two golden Cherubim. Would your wizard even know what Cherubim are? And it has been speculated by some Bible scholars that if it's true that Satan is indeed a fallen angel, then he is probably from the species of angel known as Cherubim (possibly the most powerful of all angels, more powerful than Seraphim or any other types). The two Cherubim on top of the atonement cover are quite possibly in a kneeling position. This is also something of significance: angels who kneel before God, powerful Cherubim (like Satan) who kneel before God. In your wizard's world, is there such a thing as kneeling before any god? Is there such a person as Satan to your wizard's people? And would the Satan that your wizard has been taught about ever kneel to anyone in his world?
Here's an interesting word: "Propitiation." That word appears only in the NT. It is (thankfully) found in the KJV, but only three times (four time in the NASB). And quite oddly, it NEVER appears at all in the NIV. So if it's a KJV your wizard has, then he CAN get his hands on that word. But will he know what it means? Will he have to dig around before discovering its true meaning?
Also, the 4 Gospels (Matthew, Mark, Luke and John) were written with four different audiences in mind. Mark is the most Gentile-friendly of the four, where as Matthew requires you have some background in Jewish culture and customs. John is the most esoteric with high-end theology all throughout and Luke is very scientifically-minded (he was a physician, so he gets more medically-oriented, giving precise medical details of Jesus' healings) and he's also very female-friendly (he was open to using the accounts of women, whereas the other three Gospel writers evidently had no desire to listen to very much, if any, of the female points-of-view while writing their versions).
The Book of Acts has more than one very interesting brush with witchcraft. For him to read about those instances might be very enlightening.
The Book of Revelations mentions punishments for those who practiced witch caft.
And getting back to angels: are there angels in your world? The Harry Potter books have familiars (Harry's familiar is his owl). And when you get down to it: a familiar is really a type of demon, conveniently disguised as a regular earthly creature, who aids a witch in the crafting of spells. Are there demons in your world? Do witches and wizards have familiars in your world? Are those familiars really demons in disguise? Do the witches and wizards of your world KNOW what their pet cats and pet owls REALLY are? Does your main character have a familiar? And does this familiar know what's going on with this Bible thing that the wizard got his hands on? The Bible mentions familiar spirits (although the Bible doesn't clarify what that means). Is the discovery of the word "familiar" in the Bible also something that will be a dawning revelation to him?
Getting back to Satan. In the Book of Revelations, there is one verse that is utterly loaded: Revelations 12: 7-9. Here we read about Michael and the war in heaven,
And there was war in heaven: Michael and his angels fought against the dragon; and the dragon fought and his angels, and prevailed not; neither was their place found any more in heaven. And the great dragon was cast out, that old serpent, called the Devil, and Satan, which deceiveth the whole world: he was cast out into the earth, and his angels were cast out with him.
In this section we read that the Dragon has many names: he is simultaneously called "that old serpent" and "the Devil" and "Satan" all in one verse. So there is no mistaking who he is!!!! And the fact that he is called "that old Serpent" is used as evidence by most scholars when they claim that the serpent in the Garden of Eden is one-and-the-same with this Satan character. So once again, perhaps your wizard can do some detective work and piece all of this together himself.
I like what Kim said above: just the Bible can do it (with the Holy Spirit on your shoudler).
My feeling is, your character needs to start off his character arc as a "successful" wizard, well-respected in his world. And yet in spite of all his success he senses something isn't quite "right" with the world he lives in. Something inside nags at him. There MUST be more. And then when he finds this Bible he starts to slowly unravel what that missing something is.