I think probably a major decline might start in 30 to 50 years. But that may happen with religion in general. In 50 years I believe, if mankind doesn't destroy itself, major technological advances that will stir more people away from religion. Finding advance life forms would definitely help I think.
It doesn't really matter though as I fear most of us will be long dead before any real decline happens. I'm just happy my daughter and any other children I may have will not be influenced by this sadistic cult.