Why are metaphorical and symbolic thought so common in religion? Outside the literalist traditions, most religious people will tell you that the Bible must be interpreted metaphorically. The actual events described are not so important as the moral lesson derived from it. They will also interpret many words (ie soul, spirituality, God, faith) symbolically, as something far more mysterious the the stereotypical, vulgar definitions.
By contrast, I prefer words that are clearly defined. If a word has too many meanings, I'd rather discard it, lest I confuse people. Why is this done so rarely in religion?
One reason, I think, is because of dogma.
When it comes to people's beliefs, they change a lot. The supposed rigidity of religion aside, religious people do think for themselves, and their beliefs move around throughout their lives. But despite the change, it is important that there is continuity of tradition. Believers want to believe that they are still within the tradition. The easiest way to do this is to keep the same language, even if it must be used to mean something slightly different. They still believe in the same things, they simply interpret them differently.
And this is not to say that the new interpretation is wrong. Often times, it's an improvement, or has more historical basis than the previous interpretation. And it is not so much "moving the goalpost" as it is people genuinely having a change of mind. There is nothing wrong about changing one's mind--in fact, I'd say it's a good thing. Nor is there anything wrong with metaphor, or wanting to preserve the religious language. But the fact that this is done so systematically is indicative of dogma.
So how does this work? We take a piece of dogma in religion, something general, like "God exists", "The soul exists", or "The Bible is good". To me, it is not so important whether these statements are true or false. It is important what we mean when we say they are true or false. If by "God", we mean the world, or the conception within the human mind, then of course God exists. If by "God" we mean a conscious metaphysical being who answers prayers, then I think not. If by "soul" we mean consciousness or the quality that makes one a person, then of course the soul exists. If by "soul" we mean something that comes in discrete quantities and survives the afterlife, I think not. If by "good" we mean historically important, then of course the Bible is good. If by "good" we mean true, or interesting (to me), I beg to differ.
Now, I have little reason to prefer any of the above definitions over the others. However, in the presence of dogma, it is far preferable that we stick to definitions in which the general statements remain true. With this practice of selecting the right definitions, even I could be, if not an outright Christian, some sort of agnostic or theist. If a person takes an even weaker position than my own, he/she could easily frame him/herself as a Christian. And again, there is nothing intrinsically wrong with such a decision. But the fact that this practice is so common indicates that many people (though not all) have been influenced by dogma.