This is a bit of an off-the-top-of-my-head sort of post, so doubtless I'll say something I'll later want to retract or amend, but I wanted to pursue the theme that Scott has been emphasising in comments, ie that atheists tend to see mystical language as 'cobblers'. As this is pretty central to any religious tradition, it might help to set out briefly what is going on, at least as I see it.
Think of your muscles and all that you know about them; think in particular of your bicep, of how it works and how it moves. Now ponder the fact that the word 'muscle' is derived from the Latin musculus meaning 'little mouse'. I found this fascinating when I first discovered it. I'd always thought 'muscle' was a scientific term with a very clear and distinct meaning - which it is of course - but it started out in life as a metaphor. In other words, as the language was developing and growing, and understanding was doing the same, and as people considered the parts of their bodies they remarked upon the physical resemblance between a mouse and the mass of flesh and tendon that they used to pick things up and hence: muscle.
Now this is an everyday process, especially in science. Consider the phrase 'magnetic field' - what is a field? It's something that horses run around in, normally with a fence or hedgerows bordering it. It's a confined space or area, and that is the sense in which the scientists began using the word 'field' to refer to the area that was influenced by magnetic force. Same with a gravitational field. In other words, in the process of exploring the nature of the material world the language that had previously been adequate needed to be developed and renewed in order to adequately account for and describe what was now being understood. Crucially, the understanding itself was developing in advance of the language. Have you ever had that experience when you wanted to say something and the word was on the tip of your tongue but you couldn't get it out? And then the word comes and you feel release and 'That's what I wanted to say!' Language itself is always catching up on the human life seeking expression, it's a 'static latch' to use a MoQ term, and when it has caught up then the language is itself embedded in all the practices that humanity can invent. Language is not a transparent reflection of the world, it's a constituent part of the world itself - but a part which is also derivative from and dependent on the human life and understanding which drives it. (I might say more about this another time - feel free to read my Wittgenstein essay here.) Which is a way of saying that poets are the most creative of human beings, and that the most creative scientists need to have some poetry in their soul.
However, that's taking me away from my main point: our understanding drives the development of our language, and the language develops through metaphorical exploration and analogy, and the language slowly 'hardens' in meaning (so we know exactly what a muscle is, and for most people the thought of mice in relation to muscles never enters in). It hardens so much that people forget that once upon a time it was an image, an analogy, a metaphor. You could say that, once the language and practices associated have progressed to a certain point the poetry has vanished and the concept has been completely grokked.
What religious believers want to say about religious language is that, whilst some concepts may harden (and possibly thereby become idolatrous and harmful) the core of religious knowing can never be expressed in anything other than metaphorical language. This is mystical speech: the Tao that can be spoken is not the eternal Tao. Once the language has been developed and understood the creative (poetic) flow of understanding has dried up, not necessarily to bad effect, but certainly in such a way that the ongoing exploration is impeded if the language is made absolute and unchangeable. In other words, what lies at the heart of the religious sensibility is not a conceptual truth but a relationship with reality as such, which is inexpressible in words. Or, perhaps better, it can be expressed in words, so long as the words are not mistaken for what they are pointing to, so the words must be consistently negated and affirmed in turn. (This is what I have learned from Denys Turner, amongst others, and I wrote more about it here.)
In other words, whilst it might sometimes be possible to grok muscles or gravitational fields, we will never be in a position to say that we have grokked God. Religious language is always on the boundary, on the cutting edge, always provisional and open to change. Yet, in just the same way as the understanding goes out beyond existing language, so too is the understanding fully in play in the religious sphere - in fact, I would say that the understanding is exalted in the religious sphere. Mysticism is not the abnegation of reason, it is the apotheosis of reason, where it is possible to understand reason more fully than ever before - to grok it, no less.
A final thought. Most of what is of human value emerges from that understanding which is beyond language. All the most wonderful things in human life were first conceived in imagination, as a faint glimmering or stirring in the soul, before they took material form and expression. To not see this, to see religious language (and many related fields like poetry) as sentimental 'cobblers' requires a very thorough grounding, training and education in certain mental practices and traditions. It results in what I have called 'asophism' - a form of aspect blindness, and I can't help but conclude that it represents a severe diminution of human life and potential.