The movie tries to bring into attention the essence of Orthodox Christian belief - the reunification of man with God, which is done through repentance and unceased prayer, in humbleness and ascetic struggle. According to the same beliefs, God is the one who works this union in co-operation with the man.
The main character, Fr. Anatoli, was inspired from the lives of some Russian "fools for Christ" - men who tried to hide their sanctity and keep their humbleness behind a mask of apparent insanity - especially from the life of St. Theophile the Fool for Christ.
I think the success gained by this movie in Russia and beyond is determined by the inner identification of the viewer with the concepts revealed in it.
Nevertheless, beside it's religious significance, "Ostrov" has a brilliant image directing, with superb shots of the remote locations, and a very good and balanced script - even a surprising final turnaround, for those who may have felt the need for it. You may think of it, in a way, as a sort of "fictional documentary".
As a final note, I think the movie requires at least a second viewing, both for deepening and re-enjoying it.