There are three theories concerning the origin of Baptists:
This theory states that Baptists originated since John the Baptist’s ministry at the Jordan River. Although there are similarities between the beliefs of Baptists and the early church, this theory lacks convincing historical support. These similarities also do not prove their direct descent.
Anabaptist Spiritual Kingship Theory
“Anabaptists” are so named because of their requirement of rebaptism for those who were formerly baptized as infants. Those rebaptized had to do so following a profession of faith. The Anabaptist Spiritual Kingship Theory seeks to trace a spiritual relationship from the German, Dutch, and Swiss Anabaptists, as well as the Waldensians and Petrobrusians, the Henricians, the Novations, and the Donatists all the way to present-day anabaptists. The origin of Baptist from Anabaptists has been viewed as weak by many because Anabaptists differ theologically from Baptists. Anabaptists are pacifists – they don’t believe in fighting or going to war, they deny that Christ was actually from Mary’s flesh.
“Jesus Christ was conceived in Mary through or from the Holy Spirit, but was born out of Mary and not from Mary.”
Anabaptists also hold to the soul-sleep view, that those who are dead remain in the grave until Christ comes. They also see a necessity of observing an apostolic succession in the administration of baptism.
English Separatist Descent Theory
This view states that Baptists originated from 17th century English separatists. They believed that believer’s (those who are old enough to believe) baptism was the only valid form of baptism. They believed the church should consist solely of regenerate members.
In 1611, John Smyth founded the first Baptist church in England. These Baptists believed that Christ died for every single individual, thus making salvation possible for everyone. Because of their views concerning general salvation, these Baptists came to be known General Baptists. Particular Baptists, named for their belief in Christ’s death atoning only for the elect (meaning salvation was not possible for all people) arose in England in 1630. In America, the Baptist church was founded in 1639 by Roger Williams in Providence, Rhode Island. It spread throughout the years, and in 1730 the first Baptist church was founded in the south. These churches also held to the general atonement view that Christ died for every individual. By the time of the American Revolution, Baptists were the 3rd largest denomination in the colonies (as they appealed to power and lower-class citizens) and today are the largest denomination in the United States.
source: “The Complete Guide to Christian Denominations” by Ron Rhodes