Terry Eagleton's quip that reading Richard Dawkins on theology is like listening to someone "holding forth on biology whose only knowledge of the subject is The British Book of Birds" is a funny and memorable contribution to a debate that is rarely amusing and frequently forgettable. Whether you agree with the charge or not, the complaint is of a kind we have become very familiar with: disputants in the religion debate are talking past each other because they do not have a sufficiently rich understanding of the positions they stand against.

I'm very much in sympathy with this view, and this series is largely an attempt to try to find more constructive points of engagement that can only emerge if we ditch lazy and tired preconceptions about those with whom we disagree. At the same time, however, I'm all too aware that "you just don't understand" is a card that is often played far too swiftly and without justification.

Most obviously, it cannot be the case that the views of someone who is most immersed in or knows most about a religion always trump those of a relatively uninformed outsider. People who live and breathe a faith know more about it than those who do not – but this quantitative advantage does not guarantee better qualitative judgements. If it did, by the same logic, we should take the word of the earnest astrologer of 40 years' standing over the clear evidence that it's all baloney. Indeed, being deeply immersed may be a positive disadvantage, in that it might make it impossible to take a clear-sighted, impartial view. So Dawkins and his ilk are correct when they say that they are not obliged to become experts in theology in order to make criticisms of religion.

Of course, there is a level of ignorance that makes reasonable criticism impossible. But where that is the case, it should always be possible to point out what elementary mistake the critic has made. It is never reasonable to fob someone off on the basis that they do not understand: it is always necessary to explain what they do not understand. But also – and here's the rub – it's also essential to make it understandable. Rule one of intellectual engagement is that all parties must sincerely attempt both to understand others and to make themselves understood.

It has become evident to me, however, that many people, especially the religious, suffer from a kind of conceptual claustrophobia. Their beliefs are of their essence somewhat vague and they are terrified of being pinned down. Although critics often leap on this and claim that this betrays woolly thinking, evasion or obscurantism, I think that there are times when such a refusal to commit is justified.

I remember, for example, an impassioned talk I once heard by the recently sainted Giles Fraser. Recounting the story in Exodus of Moses going up the mountain to meet God to get the Ten Commandments, Fraser said: "The higher he goes up the mountain, the more the mist comes down. The closer he gets to God, the less and less he is able to see." Meanwhile, at Sinai's foot, the idolatrous masses are "running around building a golden calf, making God into a thing".

It is always possible to think there is a fog when really it's just that your glasses have steamed up. But I'm not only prepared to allow that an intelligent religious faith might have a big fat mystery at its heart, I think it must have. Only the most juvenile gods are like super-humans we can truly understand. If there is a God, it must surely passeth all understanding.

But embracing this mystery comes at a price. If, like the archbishop of Canterbury, your faith is a kind of "silent waiting on the truth, pure sitting and breathing in the presence of the question mark", then think very carefully before you open your mouth. Too often I find that faith is mysterious only selectively. Believers constantly attribute all sorts of qualities to their gods and have a list of doctrines as long as your arm. It is only when the questions get tough that, suddenly, their God disappears in a puff of mystery. Ineffability becomes a kind of invisibility cloak, only worn when there is a need to get out of a bit of philosophical bother.

Also, maintaining that some aspects of religion are ineffable doesn't mean that all are. Indeed, it entails that some are very clear indeed. Ask Fraser, for example, if he thinks God is a thing and he should answer clearly and unequivocally, no. Likewise, people should be able to give clear answers to straight questions such as "was Christ's resurrection physical, leaving an empty tomb?", even if that answer is "I don't know". Maintaining, for instance, that it is naive to read the gospels as literal history is – or should be – to maintain that the events it describes did not, or need not, have literally happened.

I need to make these issues clear now because over the coming weeks, in the name of trying to uncross some wires and get some real discussion going, I'm going to be trying to get greater clarity about just what different camps in the religion debate are really maintaining. I anticipate all sorts of objections of the kind I've mentioned: that I'm simplifying; that I'm trying to eff the ineffable; that I am being too literal minded. I want to make it clear right now that these kinds of responses won't work as get-out-of-jail-free cards. They need justification.

We also have to be willing to accommodate the fact that belief comes in infinite shades and varieties. No two people believe exactly the same thing, and that presents another opportunity for evasion: plausible denial that you believe what is being attributed to you. We have to accept that, to make progress, we sometimes have to say, "that's not quite what I think, but it may be close enough. Go on." If anything less than perfect understanding counts as misunderstanding, then everything is misunderstood.

Everyone says that they are in favour of greater mutual comprehension, but the failure to achieve it is not just a result of people not making the effort to understand. Often it's just that people refuse to make themselves understood.

• Following up on my previous post on the appeal of uncertainty, I've posted a podcast of a talk I gave about this at a Sea of Faith conference.