"The Island" is not Hell/Purgatory. The writers are doing their own thing. Perhaps we'll never find out what the island is so they can end the story in a fairy-tale like style; Happy ending to the current situation but some events still linger on; the duel between good and bad, a magical place where miracles happen etc.
A few of parallels that I spotted between the storyline and christian teachings/stories:
1. The Book of Revelations states that Satan is banished somewhere on Earth.
2. According to "Historia Compostellana", Jacob (James the Greater, Son of Zebedee) - after being beheaded, his body was taken by angels, by boat, to an island in NW Spain where a massive rock closed around his relics. (reference to the statue Jacob lives in)
3. Richard symbolizes Jesus, the link between God and people. He was tempted (wanted to accept black smoke's offer) but didn't fall into Satan's clutches.