The Amish practice of shunning–avoiding a former member in every way possible, excluding that person from both the church and community–is considered a means of guarding the Amish culture.
The idea behind shunning is based on Matthew 18:15-17: “If your brother sins against you, go and show him his fault, just between the two of you . . . But if he will not listen, take one or two others along . . .. If he refuses to listen to them, tell it to the church; and if he refuses to listen even to the church, treat him as you would a pagan or a tax collector.” Other references are 1 Corinthians 5:11 and Romans 16:17.
Mennonites today do not practice shunning, though it is still practiced by the Amish church in some communities. In fact, the issue was one of the main reasons the Amish began as a division from the Mennonites in the late 17th century.