The issue I have with "making a game about Christians" is, in addition to great points from Patriot above, that Christianity is reality, not a game. Christians live not by bread, lives, or "faith points" but on the real word of God.
While I agree, I also recall being greatly encouraged by seeing Christian principles lived out by characters in books written by Christian authors. (Samwise Gamgee in The Lord of the Rings immediately comes to mind.)
EDIT: I understand books are static while video games are dynamic, but most video game NPCs (non-playable characters) are also static. I understand and agree with your point regarding a calculable assessment of faith and the inherent faults thereof, but I also think if games are insistent on introducing "morality systems" (which, in practice, fall far short of the claims) in games, I'd like to see a game that uses a Christian standard for those systems--understanding that Christianity is not a "ladder" by one earns salvation, but rather a life lived out of the result of us receiving God's amazing grace which we could never earn.
My idea of a game where the protagonist is charged with helping protect the underground church would cast a non-Christian in the player role and have that character witness a sincere faith in almighty God demonstrated by Christian characters' words, actions, and sacrifices. If the GTA games can highlight the effects of greed, lust, and envy, couldn't a Christian game point to (though admittedly not fully represent) how Christ transforms men's and women's lives?
Again, we run into the issue of the cost of a long-form, mission-based, open world game simply being too financially demanding to be feasible, but a man can dream, can't he?
I refuse to script prayer, script miracles, and script salvation.
While I can understand the second and third points, I wouldn't necessarily take issue with the first. If a Christian character were to pray according to Biblical instruction (say, the Lord's Prayer) and in an appropriate context in a game, I wouldn't take offense at that.
That being said I have no problem with games with Christian characters (especially Christians who are not hokey bigots) or with Christianity as a realistic and revered backdrop.
And it's clear we agree on that point. Context is crucial and I'd be very interested to see a dynamic Christian character, written by an author who is a Christian himself or herself with a daily walk with Christ, in a video game setting, if only to see a departure from the stale, "Religion is bad, mmkay?" rot of modern video games.
Seriously, how many times can a Catholic priest character pose as good and later be revealed as the great evil in a JRPG? The trope is played out and I want to see something fresh, both as a gamer and as a Christian.