'Ineffable' sounds like it ought to be rude, but in fact it is an attribute typically given to God in Christian theology. (I don't know enough about the theology of other religions to say how many of them have ineffable deities; but the arguments of this essay apply to any ineffable deity.) God is ineffable because he cannot be understood through human logic.
This is actually quite a reasonable claim. God is supposed to be infinite, with perfect knowledge of all things, infinitely beyond us in power, virtue, and will. Why should anyone believe that a human intellect could understand such a being, in any way? Within Christianity, there are a number of more specific reasons for holding God to be ineffable.
One is the doctrine of the Trinity. God is supposed to be both three and one, in such a way that the three are very different but absolutely the same. This is simply incoherent in human terms; there is no way to understand it in a way that preserves both the difference and the identity. Thus, if God is a Trinity, God is ineffable, and beyond human understanding.
Another is the problem of evil. If God is omnipotent, omniscient, and omnibenevolent, why does he allow any evil to exist in the world? There are a number of possible arguments to get around this, but most suffer from serious flaws, and thus end up falling back on the argument that God is ineffable; these things are consistent for him, even though they appear inconsistent to us.
A final example is the problem of eternal punishment. God, who is supposed to be good and all-loving, is also supposed to torture people for eternity because they refused to grovel sufficiently to him while they were alive. This is not the action of a good person, at least in human terms. For it to be compatible with God's infinite goodness, it is necessary to assume that God is ineffable, beyond human comprehension.
These are not the only aspects of orthodox Christian theology that push theologians towards the ineffability of God. The Incarnation and Redemption cause similar problems, as does the problem of reconciling God's absolute power with human free will. More arcane theological concerns also push in the same direction. The net result is that I don't think you could find a single respectable Christian theologian who believes that God can be understood by human beings, or even that we can come close.
Consequences
Suppose that God is ineffable. This means that apparently contradictory things may both be true of him, although we may not be able to see any way that they could be. It is the fact that human beings cannot see how God possibly could be that way that makes him ineffable. The Trinity is a case in point. It is a straight contradiction for something to be three and one at the same time. There is no conceivable way that God could be like that, but he can be, and is, because he is beyond human reason.
But consider another possible example. If God is ineffable, it is possible that he has promised that everyone who accepts Jesus as their personal saviour will spend eternity in bliss, that God always keeps his promises, and that all devout Christians will spend eternity in torment. This makes no logical sense, but logic does not apply to God. God is not bound by mortal logic. Human beings cannot conceive how God might be like that, but that is not a problem. God is ineffable. We cannot understand the way he is.
The natural reaction to this claim is to deny that God could be that way, because he hasn't told us that he is. So, let's look at a slightly easier example first.
In the Old Testament, God claimed that the only way to be saved was to be Jewish, and to follow all of the dietary and sacrificial rules laid down in the Torah. In the New Testament, this was shown to be false, and merely something that laid the groundwork for the revelations of the New Testament. These fulfilled the prophecies of the Old Testament, and fulfilled the law. It was no longer necessary to follow the prescriptions of the old law, because it had been made over, made new, in the new law, and through the death of Christ.
The Old Testament prophecies were fulfilled in a way that no Jew would have expected. Indeed, they probably took it for granted that the followers of the Messiah would have been in favour of Jews, and wouldn't have repeatedly tried to murder them in large numbers. Clearly, they would have been wrong about that.
The New Testament also contains prophecies of a second coming of Christ. This second coming will fulfil the new law. But, God is ineffable. This second fulfilment might be as unexpected as the first. Christians might find themselves the persecuted minority, persecuted at the hands of the saved followers of their God. They might find that the rules change yet again, so that murder is permissible, adultery encouraged, and lying compulsory in many situations. They might find that all the things they thought were important were not, but that God will save them, and any number of non-Christians, based on a completely different set of criteria.
Indeed, Muslims could claim that this has already happened. Christianity has been fulfilled by Mohammed's revelation, Jesus was not God after all, the crucifixion wasn't that important, and you can have up to four wives, not just one.
Of course, nothing restricts an ineffable God to three revelations. He could well keep changing the rules for all eternity.
The natural response might be to say that God can't do that. After all, he has said that he never changes. Of course, he said that in the Old Testament, when he appeared to be the tribal God of the Jews, with a special concern for their welfare. As God is ineffable, it is certainly possible to argue that he hasn't changed, but his support for the Jews has not been noticeably effective over the last thousand years or so. The point is that, if God is ineffable, he can appear to change radically without changing at all. Mere humans cannot understand how all of these features are aspects of the same God, but they are. Indeed, they might not even be aspects. Each contradictory feature might be the whole truth, while all of the features together are also the whole truth.
The bottom line is that an ineffable deity cannot be trusted. He doesn't mean what you think he means, because you are seeing him in human terms and he is beyond that. His promises can be kept in ways that seem the opposite of what you thought was promised.
If God is ineffable, you might as well disobey all his commands. That might be what he wants.
