Is It Really Because We Have a Hard Heart That We Don't Accept Christianity?

Christians who visit us here think that because we don't accept the claims of the Christian faith the reason is because we have a hard heart. I simply refuse to believe, they think. Sometimes they express this sentiment.

But in practically no other area of disagreement is this claim true. Among democratic free speaking people we disagree about everything, and I mean everything. We disagree about which diet is best to lose weight on, and we even disagree about democracy itself! If I don't accept Atkins diet, for instance, do I do so because I refuse to see it's merits, or because I have a hard heart? No. I just don't agree with you.

Why is this different with religious claims? I don't get it. I really don't. I just don't believe, and my non-belief is as sincere as anything I hold to.