In the most technical sense, it ought to be possible to discuss God and religion without insulting someone, since what you're discussing is God or religion, and not that person. Of course, that technicality faces a number of issues.
First and foremost, concepts of God and what religion someone practices is a personal thing. So much of how we act in life, what we do, how we handle situations, etc., is largely affected by our religion. We act this way because we believe something to be true, and it becomes very uncomfortable for someone to suggest that something we're basing so much of our lives on might not be true at all. It can feel personal because it can come across as saying everything you do may be wrong, have you ever considered that? Even under the best of intentions, that often doesn't go well.
As for the matter of insulting intelligence, well, that has little to do with discussing beliefs and more to do with how one justifies them. We all have reasons for what we believe, but, let's face it, even the best of us don't always base everything we believe entirely on flawless logic or irrefutable evidence. In particular with atheists vs theists, so often faith is attacked as being equivalent to stupid, because faith, to an atheist, essentially means believing something is true despite lack of evidence or evidence to the contrary. If you believe your faith has resulted a great amount of good in your life, to deny faith seems, well, stupid. If you believe all belief needs rational or empirical justification, faith is going to seem stupid too. As for which one I believe, that doesn't matter. ;-)
Now, having taken my second major in college as religious studies, I can tell you, it's very possible to discuss to religion without insulting a person. Objective study of religion is easy to discuss. What's tricky is trying to discuss religious views that a person actually holds and doesn't want to objectify for the sake of argument. In other words, it's easy to talk about other people's religions, but you're probably going to run into problems discussing your religion or the religion of the person with whom you're talking.
Often times it's simply a matter of people who believe with every ounce of their being, whether it be mind or heart, that they are right, and if that's your starting point for a discussion about the possibility they may be wrong, well, sadly, that's just not going to go very well.