A third idea came to me that would help the underground church evade the authorities in closed countries (such as Iran, China, North Korea, others)- it is called Steganography, I learned about this in school. In Steganography a secret message can be inserted into a file such as a picture, someone could email pictures of their vacation that look completely harmless but they actually contain secret instructions, the instructions are password protected and encrypted and cannot be found without a special program. An underground church could use stenography to coordinate their activities without fear of the instructions being intercepted and then be busted by the authorities! If you would like to have a look at various Steganography programs to do this with the link is below:

A tip that came to me- as soon as you receive the file containing the secret instructions read the instructions and then memorize them and then immediately delete the file. This way if heaven forbid you do get arrested by the authorities they would not be able to confiscate your computer and read the information after getting the password out of you (under duress).

Also there is technology called PGP (Pretty Good Privacy), you could use that to encrypt your emails to the point that they are unbreakable.

Another idea- you could use an email server that is outside the hostile country so the authorities would not be able to raid the ISP and get your emails, you could use an encrypted link to the mail server which would add an extra layer of security.