On 21 January 1525 a small group in Zurich, impatient with Zwingli's deference to the city council, baptised one another as adults in a private house. That act, not the doctrine behind it, the act, is the beginning of Anabaptism, and it was a capital crime.
The name means rebaptiser, and it was a charge rather than a self-description; the movement's own position was that infant baptism is not baptism, so nobody was being baptised twice. The distinction mattered legally, because rebaptism carried the death penalty under a Roman law revived for the purpose, and both Catholic and Protestant authorities used it. Estimates of executions in the sixteenth century run into the thousands, by burning, beheading and drowning, the last chosen as a grim joke on the offence.
The Schleitheim Confession of 1527 set out the position in seven articles, and it is the clearest early statement: baptism on confession of faith; the ban, meaning exclusion from the congregation as the only discipline the church may use; separation from the world; a shepherd chosen by the congregation; refusal of the sword, including magistracy; and refusal of the oath.
What made Anabaptists intolerable was not the water. It was that they denied the state any authority in religion and refused to bear arms for it, which in the sixteenth century meant refusing the basis on which every European society was organised. The catastrophe at Munster in 1534-35, where a millenarian faction seized a city, declared polygamy and was destroyed with great slaughter, confirmed every fear the authorities had and is still the episode Anabaptists are asked about. Mainstream Anabaptism repudiated it at once; Menno Simons, whose followers became the Mennonites, wrote against it directly.