Though I’m in no position to help, I do wish good luck as a Christian myself. I’ve often thought about what would make a good Christian game, because what I’ve seen so far were complete flops in that regard. You are in general unlikely to convert anyone, because that is not what people go to video games for (so don’t go about advertising it as THE BEST CHRISTIAN GAME). You can at least make them more open to the idea.
Christian games are chores to play, and that’s the last thing you’d want. You can’t show Jesus and co. spewing out verses word for word, because people will instead feel the need to beat Jesus up*. You can’t turn it into a verse guessing game, because that will be seen as preachy and you coercing people to read the Bible. I can’t tell you how many games like these I’ve seen.
The Old Testament has the opposite problem: it has epic stories like the Great Flood, but a vengeful God. This could put people off, particularly some who would generalize the one covenant to the whole of Christianity.
You also can’t go about casting spells and whatnot, because that is associated with Paganism and witchcraft.
Off the top of my head, the best story to showcase would be the Exodus. It has the best blend of the above: a captivating plotline, use of magic and, above all else, God won’t be seen as such a grumpy man. Don’t be afraid of taking a few creative liberties; accuracy isn’t the most important here.
My conclusion: really damn hard. Most Christian games were made by people who are Christians first and game developers second. This cycle must be broken.
*: Being able to beat Jesus up wasn’t intentional. That particular game was built on a wrestling engine. The game developer himself is Christian, and he’s actually on Itch.io.