See, I still say it was Jack's dying fantasy. Only people he cared about were in that church, and in his fantasy they get what Jack couldn't give them in real life -- happiness. In fact, Jack failed them horribly, and was wracked with guilt. But in his lingering moments, he imagined a place where all the people he let down get to go off to heaven with each other. And you can tell it was Jack's fantasy, because he got Kate. If it were real, who knows who'd she be waltzing off to heaven with.
The series started out with a strong science vs. faith dichotomy. Jack was Mr. Science, and Locke was Mr. Faith. But you know what they say: there are no atheists in foxholes. Jack dies with heaven on his mind. That's why he's smiling, and that's why they zoomed in on his eye one last time.