I liked the movie somewhat. Not nearly as good as his earlier movies, The Sixth Sense and Unbreakable, but still very much an M. Night Shamalan (sp?) movie.
To answer a few questions, Mel Gibson's character was a minister (not Catholic priest) until his wife was killed in that car wreck, at which time he lost his faith in God and left the calling.
The deeper meaning of the movie is, like Nicko said, that there is no such thing as coincidence. All things happen for a reason, whether you want to believe that there is some force behind it or not is yet another question. There are many, seemingly unrelated events during the movie that all come together in the final scenes that tie this whole premise together. Mel's wife's last words, to his brother. Mel's daughter's phobia of drinking 'bad', stale water. Mel's son's asthma. All these seemingly unrelated events all play a part in helping to save Mel's family from the aliens, especially the one that Mel, himself, had injured.
Though, I did like it, I thought there were some major flaws in the movie, and unfortunately I had most of it figured out before the end, unlike The Sixth Sense, which blew me away with the final scenes.