The number of Amish has grown 84% since 1992, to a total of some 231,000. To deal with that growth–and also to escape the suburbanization that has encroached on some of their traditional rural settlements in the Midwest–Amish are migrating, buying land, and establishing settlements in seven new states: Arkansas, Colorado, Maine, Mississippi, Nebraska, Washington and West Virginia. See Surging Amish Spreading Out.
Why the church growth? With their rejection of automobiles, electricity, computers, and other conveniences of modern life, they aren’t winning many converts. But they have, on the average, five children per family. And though the children have a choice of whether or not to stay with the church when they grow up–getting to spend their late teen or early adult years sampling the outside world–a larger number of them, 85%, are staying with the church.
Strangely, contemporary churches don’t seem to credit bringing your children to faith as evangelism. And, sadly, many evangelicals I see who were raised in strong Christian homes and knew Christ for their whole lives sometimes doubt their salvation, not having had the dramatic conversion experience many of their friends claim. But God blesses the evangelism that takes place in vocation, and He is powerfully at work in the vocation of parents when they bring their children to Christ–via Baptism, going to church, the day to day teaching and example that goes on in ordinary families. Actually, I am pretty sure that this is the way MOST Christians have come to faith, so it is not to be despised.
UPDATE: By the way, I am NOT minimizing the importance of evangelizing non-believers. We need to do that. And I would say the same thing to the Amish. (I think that lame attempts to make Christianity fun are just as ineffective with non-believers as they are for youth in our church, and that presenting Christianity in its truth, in its depths, would be more effective for both.)