Mild spoilers, but no details.
Catholicism vs Tibetan Buddhism with Star Wars thrown in.
Just as Star Wars (well, the six movies and the Clone Wars cartoon Series, according to George Lucas) was really all about the Rise, Fall, and Redemption of Anakin Skywalker, so too Lost was the Rise, Fall, and Redemption of Jack Shepherd.
Which makes Season Six into all about Purgatory/The Great Bardo.
The Island turned out to be Dante's Purgatory (funny, when I read La Divina Comedia back in High School, I said Hawaii was Purgatory. Guess the Lost creators agreed with me, somewhat). Jack was destined for Heaven to begin with, but needed to purge himself (quite painfully at times) of his daddy issues and individualism to get there (ended up being a slave in service to the Island and dying that way). Six years of purging, from opening his eyes after being ejected from the plane in the bamboo grove of the first season, to closing his eyes in death in the bamboo grove in the very last scene. ALL other characters were secondary- including the most infamous Hugo, who supposedly was Jack's candidate to take his place, and Jacob, who was the protector of the Island before Jack. The real storyline, throughout it all, was Jack's rise, fall, and redemption.
The flash sideways timeline in a parallel universe, turned out to be a Tibetan Bardo for all of Oceanic Flight 815- they needed to reunite to be able to move on to Heaven. And here too, last to realize what was really going on, was Jack, meeting his father in the back room of the Church, with all the identifying symbols of the world's six greatest religions (Star of David, Cross,Star and Crescent, Ohm, Dharmacakra, and YinYang) in a stained glass behind him.