The Amish originated with a group of Protestant Christians commonly referred to as the Anabaptists, or "re-baptizers." Baptism is the Christian rite whereby the initiate is symbolically cleansed of sin and is "re-born" into the faith. Infant baptism has become widespread in part because infant mortality has historically been high. The Anabaptists believe that infant baptism is invalid because an infant is incapable of understanding the meaning of the rite. The Anabaptists do not perform baptism until the candidate is old enough to make an informed decision and to accept personal responsibility.
The Amish came into being in 1693 when a group of Swiss Mennonites led by Jacob Amman broke from the main body of Mennonites over differences related to the celebration of Communion (a remembrance of Christ's last earthly meal) -- Amman wished to celebrate Communion twice per year, while the Mennonites celebrated it once per year; the Biblical command to remain separate from non-believers -- Amman wished to adhere to this separatism, while the Mennonites intermingled with non-believers; and the washing of feet (a display of humility) -- Amman wished to practice this ritual, while the Mennonites did not include it in their ceremonies.
Facing persecution from both Catholic and Protestant Christians, Amish in large numbers eagerly took up William Penn's offer of religious freedom in the American colony of Pennsylvania. Immigration to Pennsylvania began in 1727 and continued in earnest through 1770, settlement being concentrated in the Lancaster County area.
The Amish do not have church buildings. Perhaps because of early persecution, the tradition arose of worshiping in the homes. The home that will hold services is selected on a rotating basis, so all homes are equipped for conducting worship services. You can identify these homes today by the large number of buggies present on a Sunday morning.
The Amish settled into farming because this rural lifestyle made it easier for them to keep their distance from non-believers, referred to simply as "The English." Cities and towns have more of a tendency to become melting pots. As their numbers grew, Amish settlements were established in Ohio, Indiana and many other states, as well as in Canada. The establishment of new communities is ongoing.
The Amish speak a Low German, similar to Pennsylvania Dutch, among themselves. High German is used for church services, and English is spoken with outsiders.
The Amish split into a number of divisions, including the conservative Old
Order Amish and various more liberal groups.
Membership in the main Amish church, the Old Order Amish Mennonite Church is not
reported. The other Amish groups are relatively small. Probably the total of all
Amish groups would be on the order of 100,000 in 22 states, including about
45,000 in Ohio and smaller numbers in Illinois, Indiana, Pennsylvania, New York,
etc. There are about 1,500 in Ontario, Canada. Almost all members are
descendants of Amish parents. Converts are believed to constitute less than 10%
of the total membership.