Other times, people mean that we cannot fully comprehend God. There are some aspects of God that are easier to grasp, but we have to constantly remind ourselves of the majesty and mystery of God. I'm good with that.
But sometimes people use this phrase to avoid saying anything too specific or too definite about God. Now, this can be a reaction to the overconfidence and closed-mindedness of their own background, where people never ask questions and never think too hard about what they're talking about when they talk about God. So saying "You can't put God in a box" ends up being a response to anyone who says anything too definite or specific about God. I'm not good with that.
Here's the rub: saying "you can't put God in a box" IS in fact saying something very specific and very definite about God. Saying "you can't say anything too specific or too definite about God" is in fact saying something VERY specific and VERY definite about God. So sometimes people say something like, "Well, everybody has a different view of God but it's fair to say that there's some kind of basic reality or deity underlying it all, and we all basically believe the same thing." What they fail to note is that this is as definite, specific, and certain as any hard-core fundamentalist.
Christians need not be arrogant, but we can be certain. God has revealed himself in a very specific and definite way. It is not arrogance but humility that takes God at his Word that he is a very specific and definite kind of God, even as we acknowledge that his ways are higher than our ways and that we can never exhaust the mystery and majesty of God. And it is not true humility but arrogance masquerading as humility that doesn't want to say anything too specific or definite about the God who has most fully revealed himself in Jesus.
As Hebrews 1:1-3 puts it, "God, after he spoke long ago to the fathers in the prophets in many portions and many ways, in these last days has spoken to us in his Son, whom he appointed heir of all things, through whom also he made the world. And he (Jesus) is the radiance of his glory and the exact representation of his nature, and upholds all things by the word of his power."
We speak not as initiators, but as responders to the Word who brought all things into existence, who sustains all things with his power, and whose blood cleanses us from all sin. We dare not fail to speak of this living Word, the one who has the words of eternal life (John 6:68).
Here's how John Howard Yoder talks about "godly arrogance": "I do not intend to challenge the need for growth in modesty and in cultural perspective, but I do intend to challenge the tendency to make a hobby out of a corrective. Today's most urgent need is no longer perspective and modesty. What today's world and church need most is a recovery of the missionary arrogance of the New Testament church. To arrogate (the verb from which we get the unpopular adjective arrogant) means to make claims for oneself or for one's cause. If the claims we make are for ourselves, then it is understandable why we need to overcome our arrogance. But if the cause for which we are making claims is the cause of the one true God, then anything short of absolute demands is unfaithfulness" (Radical Christian Discipleship, p. 44)
So do we need modesty and humility when we talk about God? Certainly. But proper humility and modesty means that we speak where God has spoken, that we not allow our pride, our culture, or awareness of our own limitations force us to stay silent.