Beth - All I know (and, regarding religion, that's not a lot) is that Seven appears a lot in the Bible. There are 7 churches, 7 Spirits, 7 stars, 7 seals, 7 trumpets, 7 vials, 7 personages, 7 dooms, and 7 new things - apparently in the Book of Revelation alone. To give numbers, its used more than 700 times in the Bible and 54 times in the Book of Revelation. It's also used more than any other number in the Bible other than One. 7 Days of Creation, 7th Day of Sabbath, 7 Weeks between Passover and Pentecost, 7 Day Feasts, Every 7th year the farms left fallow, Every 49th year a year of Jubilee...
I'm no expert, but one would presume all these references to 7 are connected and have some significance.
Apparently, Seven is a sign of completeness as it merges "the perfect world number, 4 and the perfect divine number, 3." Four represents "the four square Earth" [the four compass points?
), and Three the Trinity, so that Seven represents the union of Earth and Heaven, and thus completeness.
Disclaimer: I don't believe the above!
Though I guess it makes as much sense as anything else, and I do think that all those Biblical 7s must have some meaning behind them. 7 could simply signify completion because God seems to favour it so much.
In any case, accepting the Biblical 7 to be a number of completeness... why did Tolkien seem to pervert this? As I've mentioned above, the 7s in Tolkien seem to fall or be partially corrupted.
In the Case of the Valar, the Valier and the Stars, 7 probably does reflect completeness and divine favour though. As
davem said, the Numenoreans picked up on this too - 7 circles, 7 day week, harmony with God.
Consciously Christian in the revision, eh?