Suppose I am convinced that God exists. Then if p is true, God believes p. So it seems that whenever I have the right to assert p, then as long I should also be willing to say: "And God agrees with me." But to say that God agrees with me sounds awfully arrogant!
I suppose some of the apparent arrogance comes from the implicature that I have independent evidence that God agrees with me—a special line to God. But of course in the typical case, my evidence that God agrees with me about p just is my evidence for p (plus my evidence that God exists and that I believe p).
Or maybe it's that one implicates certainty. (Why? Is it because there is a stereotype that when people make claims about God they are certain of them?)
There is a similar impression of arrogance one conveys when one says: "I have the truth about p." Yet, of course, if one is justified in believing p, one is typically justified in believing that p is true, and hence that one has the truth about p. Again, maybe the issue is that saying one has the truth implicates certainty?
There is, indeed, something odd about claiming that God agrees with one or that one has the truth on a subject where one has only a weak opinion. I am about to have random.org choose a number between 1 and 10, both inclusive. I think that the number will be smaller than 10. But it would be odd to say: "God agrees with me" or "I have the truth on that." Yet, my evidence that I have the truth on the number being smaller than 10 is almost as good as my evidenec that the number will be less than 10, and my evidence that God agrees with me is very good, given that I have very good evidence that God exists. (Oh, and I was right. The number turned out to be 1.)