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.