Shortest possible answer: "Says you."
It was Christians, among others, who declared that God is beyond understanding, after failing to reconcile His destructive actions in the Bible (and other catastrophes since) with His supposed existence as an omnipresent, omniscient, omnipotent and entirely benevolent being with an interest in humans. It's one of the many approaches to theodicy, and probably the most dismissive and least satisfying.
Remember that this is the being from whom fundamentalist Christians attempt to take their entire moral code, word for word. Yet they admit that nobody, let alone them, understands why God supposedly commits such atrocities or allows them to happen. If they can't understand this side of Him, they really have no understanding of God's own moral code at all, and they're flying blind.
One reply would be that God's moral code applies only to God and not to humans. Some of God's supposed instructions to humankind are pretty clear and just about sensible (like the Ten Commandments), but other times he instructs armies to wipe out entire races, so it's not as if His unexplained lapses in benevolence never affect us.
In short, as long as Christians declare that we cannot understand the God of the Bible, using Him as the ultimate arbiter of morality (or anything else) is a shot in the dark.
I'm honoured to help. I think I can be properly impartial, because I don't agree with either of you; I don't believe in free will at all. We can do what we want, but we can't choose what to want.
Just a quick note before I begin - isn't "Christian theist" partly tautological? Is it possible to be a Christian and either a deist or an atheist? How would a person of either combination reconcile the supposed miracles of Christ?
Plus, a quick glossary for those not used to this kind of discussion: omni-max means omnipresent, omniscient, omnipotent and omni-benevolent all at once, as Christians claim God is. P1, P2, etc. are premises from which conclusion C is derived. If the combined P doesn't lead to the C, it's a bad argument.
You can do better than an unknown hypothetical OM/FW harmoniser. There are known hypotheticals out there. For free will to exist, let's go large and say it must be possible for any human to make any choice at any time, even if that choice may not agree with a god. One solution which also allows an omni-max god is that all choices agree with that god; the nature of benevolence is that all human choices, regardless of the suffering they may cause, are ultimately beneficial. The god has used its infinite mind and intuition to predict all possible futures, and has found (or declared) all of them to be good. This is a potential solution to the Problem of Evil as well.
Therefore I don't think there's a deductive path to a contradiction between an omni-max god and free will, so if you're going to argue it anyway evidential and probabilistic arguments are the way to go.
To add my own spin, the fact that there are answers to the tough questions like supposed free will and the Problem of Evil does not help Christianity as much as you'd think. The problem is that there are too many answers coming from the one religion. The PoE is the better-known example. Answers to that can involve exempt demons, free will, "mysterious ways" and the nature of evil itself. It would sound better if Christians settled on one answer, because the current multitude demonstrates that nobody actually knows.