Who are the "extreme Anabaptists" who believed that regeneration makes people incapable of sin?

While reading Louis Berkhof's *Systematic Theology*, I noticed an interesting tidbit about the beliefs of "extreme Anabaptists" on regeneration. He argues: > [Regeneration is not] a complete or perfect change of the whole nature of man, or of any part of it, so that it is no more capable of sin, as was taught by the extreme Anabaptists and by some other fanatical sects. ([4.6.C](https://www.ccel.org/ccel/berkhof/systematictheology.vi.vi.html)) From this I gather that these "extreme Anabaptists" believed that at least some part of the "nature of man" was completely, perfectly transformed in regeneration, making the regenerate unable to sin. This does seem outside the norm for Anabaptists, and it makes me wonder: - Who taught this, and when? - Did they consider regeneration to be something instantaneous (as in Reformed theology), or a process (like sanctification)? - Did they believe that the whole nature of man, or only a part of it (soul? spirit?) was transformed?