Christianity is an irregular verb par excellence (as too, of course, are all the other world religions). To speak it and understand its hopeful message you simply have to learn them, live them, always use them in the context of the world in which you find yourself. They are never reducible to a set of simple unifying, rational rules.
Religions as vinegar tasters - there's a Taoist painting of Confucius, Buddha and Lao Tsu tasting vinegar; only Lao Tsu is smiling and enjoying the vinegar for what it is. The vinegar represents life, the world as it is. Another article by Jeff Lilly explores the idea of the vinegar tasters.
Religions as software - if your brain is the hardware and your mind is the operating system, religions are the software installed on it (and sometimes it's really difficult to uninstall them). My article, Religions as software, explores this idea.
Religions as ex-girlfriends - Al Billings' hilarious article (update: sadly no longer available) explores the idea of religions as ex-girlfriends, which means they naturally have opinions of each other:
[Wicca] complains about your “kablahblah” and rolls her eyes while mumbling about patriarchal power schemes. She can’t stop talking about Roman Catholicism and how wrong she was for you… in fact, she seems pretty obsessed with her sometimes.Religions as explanatory tools for various situations - like why shit happens (surprisingly accurate); why your web page cannot be found; and of course, how many adherents it takes to change a lightbulb (there are Christian lightbulb jokes, Pagan lightbulb jokes, Jewish lightbulb jokes, Buddhist lightbulb jokes, and there may be many others that haven't been discovered).
Religions as cities - this one's been popular ever since someone dreamed up the heavenly Jerusalem, and Augustine burbled on about the City of God. Nevertheless, not a bad metaphor; different denominations can be different suburbs. As Evelyn Underhill famously said, ‘the Anglican Church may not be the city of God but she is certainly a respectable suburb thereof’. Andrew Brown has a lovely article on religions as cities. If Christianity is a city, is Paganism another city (possibly with more trees), or is it the surrounding countryside?
Religions as receivers of frequencies - it occurred to me the other day that each religion has its own frequency for tuning in to the numinous, and that in between the frequencies, there is static (but perhaps one day a new radio station will appear there). Or perhaps one religion is tuned to light, another is sound, and another is radio waves, and so on -- so each religion is a different type of receiver for detecting the emissions from the numinous.
Religions as colours - each religion has a different set of colours representing the philosophical and cultural ideas within it.
Religions as rhizomes or river systems - Deleuze and Guattari's idea of the spread of ideas as being like the growth of rhizomes could also be useful here. Similarly, religions are discourses, so the idea of discourses as rivers could also be useful. R Diaz-Bone (2006) describes discourses as an 'expression, indeed part of a certain social praxis, that already defines a certain group of possible texts, that express that same praxis, indeed can be accepted as representatives of that same praxis.'
Religions as trees: Tolkien described the Catholic Church as a big tree growing into time with its roots in eternity; and regarded the Protestant Reformation as an attempt to chop down that tree, with all its interesting gnarly bits, and start again with a new sapling. Regardless of what you think of his particular religious politics, it's a great metaphor. Trees grow in a particular place and are nourished by the soil and shaped by the winds that blow, so each religion is shaped by its environment; but all trees are recognisable as trees and have some features in common, by which we can compare them, so this metaphor gives you essence (the quality of treeness) and particularity (type of tree, environmental conditions).