I didn't watch the series, just the final episode and some dribs and drabs.
I saw it as they all died in the initial crash and existed in some in between nether world, until everyone could unload their worldly baggage. Once Jack made the final step and met with his father, he was free to join the others and they could all move on.
All except for Ben. He didn't go inside the church because he wasn't dead. In the closing shot of the beach and aircraft debris, there was one solitary set of footprints wandering around the wreckage. Maybe Ben's. Might explain why he was so prominent in everyone elses story line. Just guessing.
Meh. I'm sure there will be plenty of theories to follow. After watching the interviews, I'm not even sure the authors\ scriptwriters knew what they were doing, where they were going or what the ending meant. They took it as far as they could, then wrapped a hasty, sloppy bow around it all and sent it away.
That's just my :2c: given what little I know about the show. I'm sure someone will have something more concrete, sensible and interesting than my idea.