Ineffable means beyond expression, beyond words. Mystical and religious experiences are ineffable. You cannot talk about them; at best, you can talk around them. They are not perceptions, but transformations of perception. They have no content, but throw an altered light upon whatever ideas you have. The experience is formless, yet informs everything; empty, and yet fills everything with meaning.
Yet, if these experiences really are ineffable, why do people try to talk about them at all? The content of an ineffable experience is purely emotive, an experience of certainty which is then attached to whatever happens to drift through your mind. It's a lot like cocaine, which makes your believe that everything you think and do is the right thing. I tried it once and found myself wondering why anyone would ever do that to themselves deliberately. Rational judgement becomes impossible. I suspect that certain religion experiences operate in a similar way. Everything seems right, true, and perfect. You are willing to entertain wild ideas. This is not a bad thing unless you do not go back and cull out the nonsense that creeps in. And a lot of nonsense will creep in.
Years ago I had a prolonged period of mystical satori (not drug induced) and I can tell you that if you're not careful you can amass a lifetime's worth of bullshit in a very short time while in this state. It is a condition of apparent enlightenment--and if you are enlightened, you couldn't be wrong, could you? I knew I had gone too far when I started entertaining the significance of signs, wondering whether a title of a book I happened to unpack that day had some special meaning. I laughed when I found myself thinking this. My sense of humour probably saved my sanity. One of the common traits of cult-leaders and fanatics is that they lose their sense of humour. The conviction of enlightenment can make you very dumb. As the Bhuddists say, if you meet the Bhudda on the road, kill him. This is not to say that I learned nothing; I came to value optimism, imagination, and empathy more strongly, mainly because I struggled to preserve the essence, rather than the outcome, of the experience. But I also experienced what it must be like to be born-again, and understood why so many of them seem to be mad.
My main conclusion from that experience was that idolatry was a sin because any representation of God was false, and this included written, spoken, or imagined representations as well as those made with physical materials. The experience was truly ineffable, it was pre-cognitive. There are two traditions in theology, apophatic and kataphatic. Apophatic theology is negative theology; you can only say what God isn't. Kataphatic theology makes claims about God. Mystics hold to apophatic theology. Some theologians have argued that the Neo-Platonists corrupted Christian theology through their temptation to make positive claims about the unknowable. This is why debates about how many angels could dance on the head of a pin came to be held in such contempt--they had missed the point entirely. Modern evangelical and fundamentalist Christianity is kataphatic, which means it completely ignores the Christian mystical tradition. It consists solely of claims about existents, claims about the material world, or about an alternate material world. Their God is basically human, with human intentions, desires, needs, and goals, which apply to and affect the world of matter. The language of metaphor is read in the descriptive mode; poetry is read as a simple manual of instructions. It is essentially a mundane theology, which harps upon arbitrary and rather extraneous ideas which believers fixate upon during their period of false certainty. As such, it is spiritually tone deaf.
My friend Pat told me yesterday that he believes that the atheism of Sam Harris, author of The End of Faith, is in fact apophatic theology. An apophatic theologian might claim that he believes in God, but he does not believe that God exists, or like Harris, that he does not believe in religion, but he does believe in spirituality. When the Romans first entered the Hebrew temple and Holy of Holies, they found an empty room, and concluded that the Jews were atheists. It is this apophatic tradition which lead to the ban against idols, the ban against speaking God's name, and Jesus' use of parables and indirect modes of speech. You cannot talk about God, you can only talk around an ineffable experience. As Lao Tzu said, "The Way that can be spoken is not the true Way." The ban against worshipping other Gods becomes a ban against worshipping any God when you consider that the Hebrew God eventually became a God without a name, a face, a place in the world, or any known characteristic. Another perfect expression of apophatic theology is Socrates claim that "All I know is that I know nothing." Socrates, too, was condemned as an atheist. Apophatic theology is indistiguishable from atheism because it has no object. The only way to know God is through love and the study of nature. Even the word God is of little use, because it means nothing. There is no dogma, only a method. The Bhudda holds aloft a lotus flower and smiles; the masters smile, bow, and leave, and the rest wait patiently for the explanation. To those who know, no words are necessary; to those who don't, no words will suffice.
The essence of the ineffable experience, and of apophatic theology, is that it makes no truth claims, and therefore can never contradict science. It is, after all, just a feeling, albeit a well-adapted one. I suspect that the true heirs of Jesus and other mystics are not the believers, but scientists, driven by their love of truth and their fellow man, and their wonder of the universe. During my own mystical experience years ago, I felt for the first time like I was reading the Bible from the inside. Some of it is truly profound, and much of it is just plain awful. But the experience didn't lead me to the church. It led me here.