Traditionally, Christians used to complain that some folks (e.g., atheists) suffer from a difficulty in believing in invisible realities. While there may be such an affliction, I think in a way it can be harder to believe claims about visible things. Consider, for instance, the difficulty a lot of people experience in believing that miracles have occurred out in the extra-mental physical world (some people are much more open to believing in miracles of inspiration than in physical extra-mental miracles), or the difficulty in gaining acceptance for philosophical theories that make empirical predictions, even if these empirical predictions are compatible with everything we know. In regard to the latter, I am thinking of three examples:
- Certain versions of libertarianism which entail that there are brain events that do not supervene on events that can be explained by physics.
- Versions of General Relativity Theory that entail that our world has foliation by spacelike hypersurfaces (e.g., Crisp's presentist General Relativity).
- The claim that no future pope will ex cathedra teach a doctrine that contradicts earlier infallible teaching of the Church's magisterium.
I think the issue is simply that it is pretty easy to believe propositions that have the property that even if they were (perhaps per impossibile) false, one wouldn't know it. In believing such propositions, there is no risk of being proved wrong. But it is harder to believe propositions where the belief involves a certain risk—i.e., propositions which are such that if they were (perhaps per impossibile) false, one might be proved wrong.
Similarly, it may be easier to believe propositions that have no practical consequences for us over propositions that have practical consequences. Consider for instance that a lot of people find it a lot easier to believe that God is a Trinity, despite all the paradoxes inherent in this doctrine, than they do to believe that contraception is always wrong, which doctrine does not involve any serious paradox, though it is counterintuitive relative to our culture.