there is "behind", "underneath" or "at the back" of everything something that is the really real, the real truth, the essence of religion/philosophy etcas my friend Andrew Brown wrote in a comment in reply to me (and he is rightly critical of and resistant to this tendency).
I think it is possible to draw parallels between, and gain illuminating insights from comparing, different religious traditions, without saying that they are the same or that there is an essential underlying truth which they both fall short of. For instance, it is fruitful to compare Christian contemplative prayer with Buddhist meditation, without saying that they are both imperfect expressions of some lost original. However, I do think that all these practices and ideas are grounded in some actual psychological experience which is common to human beings because we are finite entities in a physical world yearning for epistemological transcendence. I have tried to draw such comparisons in a previous blogpost which was a reflection on a paragraph from his previous blogpost.
I think I still failed, and still fell into the trap of assuming an underlying reality to which different traditions point, though. It's easier to make a distinction between practices than it is between abstract ideas.
However, I know that you can't simply transfer symbolism from one tradition into another. For example, the Unitarian interpretation of Hanukkah seems rather different from the traditional Jewish view of it. Similarly, someone once asked me to "do a service about Wicca", and I found that it was impossible. You can't understand what Wicca is like and why it is attractive without actually participating in it fully, and that means being initiated and really immersing yourself in the symbolism, the mythology and the seasonal festivals. Anyone trying to experience a taste of Wicca through the medium of the Unitarian hymn sandwich would come away sadly perplexed; and if you tried to do a Wiccan ritual in a Unitarian church, you'd end up watering certain things down to the extent that it just wouldn't be Wicca any more.
I get different things out of being a Wiccan than I do out of being a Unitarian. Both are necessary to my spirituality, but in very different ways. And you can't translate the one into the other. In order to enjoy Unitarianism, you have to be immersed in its symbol-systems and assumptions about how the world works. You can find places where Unitarianism and Wicca overlap; you can use ideas from one to illuminate the other - but they are not interchangeable.
Similarly, you can't assume that the Runes are a "Norse version of the Tarot". The Runes are a symbol-system in their own right, with their own cultural and historical background, which is very different from the cultural and historical background of the Tarot. It might, however, be interesting to do a rune-reading and a Tarot-reading on the same issue, and see how the two systems described it.
So I do agree with Andrew - we should not assume that all religions are imperfect views of the same underlying mystery. They are each unique and beautiful symbol systems, and it reduces them to something less than they are to try and shoehorn them into a "Theory of Everything". That's not how religious language works; it is metaphorical and embedded in its particular context.
I think the metaphor of religions as languages is helpful here. Sometimes it is possible to translate from one language to another, but sometimes the metaphor and symbolism is lost. Some authors cannot be translated from their original language because the imagery is lost thereby. Sometimes there is a word or expression in one language which has no satisfactory equivalent in another language. This is because its network of connotations and denotations is different. This is even true for closely-related languages like English and German, and is even more true of distantly-related languages like English and Sanskrit. Words used for complex concepts in Buddhist and Hindu thought have often been mistranslated or poorly expressed in English. The same thing happens when you try to import a practice or a concept from one religion to another. Especially if one of those religions is a conscious attempt to recreate an initiatory mystery tradition, and the other is an heir of the Enlightenment impulse to de-mystify everything. Even if you try to translate the concept of the real presence in the Eucharist, or different models of the atonement, from one form of Christianity to another, confusion and miscommunication can result. In order to understand a concept or practice thoroughly, you really do need to be immersed in the tradition from which it comes. That's not to say that a reasonable level of understanding cannot help to illuminate one's own practice and theology; but we should beware of facile borrowing just for the sake of innovation or "being inclusive". Some attempts at including elements from other traditions can seem patronising or inept, or just plain fluffy. (I've seen this happen.)