Falun Gong claims 80-100 million adherents, well outnumbering Judaism, Sikhism etc.
An odd one I’ve not really considered before is “Spiritism”. It claims up to 40 million adherents, mostly in Brazil, and so is also larger than Judaism, etc.:
Its books:
If one doesn’t read Arabic, is there an English translation of the Koran that is considered acceptable by Muslims?
Which version of the Bible would fellow Dopers recommend for Flik’s collection? All of them?
I know that at least some Muslims say no there isn’t, that only Arabic is acceptable. I don’t know whether this point of view is shared by all Muslims or not.
It depends on Flik’s particular purpose(s). Authoritative reference? Historical curiosity? Personal education?
Many Bible publications are clearly not worthwhile except for those who like to accumulate one of everything. But which one is “the best”, depends.
Codex Sinaiticus.
More seriously though, two of them. King James for the poetry and New Revised Standard for accuracy. The King James is nice because of its effect on contemporary language and thought. The NRSV is nice because it isn’t as bound by patriarchal tradition and modified itself in light of recent manuscript doscoveries.
‘All of them’ would fill quite a shelf.
Like **DavidwithanR **says, it depends. The KJV should be in there for historical value and the Elizabethan English. For modern translations, there are a number that people are partial to - the Jerusalem Bible and the New International Version come to mind, but there are many more.
I’m not a very good judge of translations: I got hold of a copy of the New English Bible 47 years ago as a new Christian, and basically did a mind-meld with it. I can use another translation if I have to, but the NEB just fits my brain in a way that no other translation can come close to. So I’d recommend it, but I’m biased as all get out. 
I’m ELCA but I would throw in The Book of Concord as well. It isn’t an easy read but it is worthwhile especially for those looking at us from the outside.
Aside from the other versions mentioned, you’d probably want a Catholic bible which has seven deuterocanonical books that aren’t in most Protestant versions.
The Urantia book would be an interesting choice.
“Apocrypha” can refer to the parts of the Bible that are accepted as canonical by Catholics but not by Protestants (Staff Report). Since they are considered part of Scripture by a substantial number of Christians, they should definitely be included in the OP’s collection.
“Apocrypha” can also refer to books that never made it into the Bible (Straight Dope). I’d say there’s no reason to include such works unless the OP is trying to be very inclusive in his definition, in which case books such as those suggested by Ulfreida, dalej42, and kopek have more claim to be included, even though they have nothing like the same status as the Bible.
I approached philosophy by way of martial arts and approached religion by way of (Far Eastern) philosophy. In addition/contrast to the Tao te Ching I would suggest The Analects of Confucious. For that matter, I would even suggest Sun Tzu’s Art of War (and, for comparison to that, try Machiavelli’s The Prince – but these would be wanderings into philosophies rather than religion).
Around the mid-1900’s a Japanese scholar was visiting a Belgian professor and their conversation eventually came around to the Belgian essentially asking, “But without a scripture, how do you teach morality and ethics?” The scholar’s answer to that was to set in writing the norms and values and attitudes of the samurai that the Japanese simply seemed to follow without the need for a catechism or intentional instruction. Bushido: The Soul of Japan would give you the philosophical foundations of Japanese culture while something like The Kojiki would provide a historical exposure and context of Shinto, the native religion of Japan. And, if you’re going to get into Bushido, then I would (naturally) recommend Musashi’s Book of Five Rings.
Of course others have listed the Hindu scriptures, but they would the roots of Buddhism like the Torah is the foundation of Christianity and Islam. The Teaching of Buddha is found commonly enough in Japanese hotel rooms; it seems* to be Asia’s answer to the Gideon Bible and, similarly, you’re welcome to take it for free. I happen to have a copy from I-dunno-where. If not that, then The Buddhist Catechism is free as an eBook.
For that matter, I also found, free as eBooks…
The Edda and the Elder Eddas (Norse)
The Enuma Elis (Mesopotamian)
The Zend-Avesta (Zoroastrianism – ancient Persian)
The Kitáb-i-Íqán [the Book of Certitude] (Bahá’í)
I also found quite interesting a free eBook called American Hero Myths: A study in the Native Religions of the Western Continent which shows (repeatedly) the transformation of vague allegory into personified aspects of nature into specific supernational entities into a pantheon of deities, each occurring separately but following similar phases and twists.
Since The Gita is a subset of the Mahabarata, I would encourage you to simply get the larger work. Now that you’ve got a hole in your collection, I would suggest that, since you’re including the Yoga Sutras, you might consider including the Kama Sutras, as well.
–G!