Shunning is the practice of ostracizing the apostate. If you leave the religion, adherents will cut off all interaction with you. Shunning is practiced by highly authoritarian religions like the Amish, the Baha’i Faith, Jehovah’s Witnesses, and Scientology, and serves as a powerful sanction against religious rebellion. Shunning may be very traumatic for the victim, especially when he has never been encouraged (or even allowed) to form any close relationships outside the Faith.
If your religious authorities are so insecure, and so controlling, that they require you to cut off all contact with anybody who has actually defied their authority, your religion is fucked up. And if you will agree to shun your friend, your child, your spouse, or anybody else because she became (for example) Mennonite instead of Amish, you’re pretty messed up yourself.