They were at St. Andrew's Cathedral near downtown Honolulu. This is the same church that Charlie visited in Season 1, and where Mr. Eko worked in Season 2. Apparently they were there most of the day, with scenes outside in a courtyard. By the time I arrived, they'd moved indoors... usually meaning the end of the road for looky-loos.
But, they were filming in a doorway, affording a surprisingly good view from the camera's POV, even from the sidewalk. And I saw the flashback's star appear behind a clapboard: Henry Ian Cusick, or "Desmond."
He was neatly shorn. He was in a long grey robe. He was talking to another man, a superior perhaps, in a black robe and skull cap. And as the scene wrapped (and they closed the door), I saw "Desmond" get down on his knees, palms together and up, as if to receive communion.
So... has Desmond lost his faith, or tried to find it again? Very, very interesting! Source: Ryan@The Transmission