Ancient Walls
The Great Wall of China (which contrary to popular myth is not visible from outer space) was built to keep people out.  Nowadays it has the opposite effect—attracting tourists to China to come see this Great Wall.  Communist China raised up “walls” to keep Christian missionaries out.  This also had an opposite effect—underground churches flourished and native missionaries sprung up.  Another religion foreign to China, rushing into the spiritual vacuum created by Communism, was Buddhism-- which has also experienced a surge in popularity.