On Fröhlich-inspired Nazarene separatism

The library where I work recently discarded some 800+ old books (mostly in English and German, some dating before 1900). I only snagged one, a book published in 1931 by E. H. Broadbent, and published by Pickering & Inglis (London) titled The pilgrim church: being some account of the continuance through succeeding centuries of churches practising the principles taught and exemplifed in the New Testament. I was leafing through it yesterday and found that it contains some material on the Nazarenes, or the Apostolic Christian Chruch as it's known in the United States. This is from page 345:

Owing to his own experiences, Fröhlich wrote with unmeasured condemnation of the formal religion prevailing in the great Churches, Catholic and Protestant, and the Nazarenes generally are unsparing in their denunciation of what they believe to be contrary to the teaching of the New Testament. Among them a Lutheran church may be described as a "den of thieves", while many of them seem hardly to believe in the possibility of salvation outside their own circles. This exaggeration shows itself in Fröhlich's teaching.

The entirety of the text is available at Project Gutenburg Australia. The part on the Nazarenes is on pages 342-346 (in the chapter titled Russia). Fröhlich is also mentioned on page 373.