Why do you think it was?
Just the way it was presented to us :/
Christian said to Jack "Everyone dies sometime kiddo. Some before you son, some long after you." - with reference to everyone else at the church. So if they died at different times then surely they were alive on the island.
He also said specifically to jack that the life was real, the people in the church are real, and then Jack remembers lying in the bamboo forest watching the Ajira plane leaving and finally dying. (So damn beautiful, brings a tear to my eye just recalling the ending!)