The following quote is from a footnote in the book, “The History of the Christadelphians”, page 27, by Andrew R Wilson, who comprehensively researched the emergence of the Christadelphians as a denomination between 1864 and 1885.
“There was considerable credal flexibility amongst the Baptised Believers, especially in the period before Robert Roberts became editor of The Ambassador of the Coming Age diminishing between 1864 and the death of John Thomas, much less between 1871 and the Inspiration Division (see Chapter V), and hardly at all after 1885. In the earliest of these periods, this credal flexibility concerned doctrine as well as ecclesiastical procedures. William Norrie’s Early History is replete with references to a wide variety of liturgical practices, ecclesial officers’ titles and functions, the organisation of services, the titles by which the ‘ecclesias’ were known, the credal formulae they accepted, the use to which such creeds were put, the practice of re-immersion upon a deeper understanding of the gospel, and the attitude to practical matters such as politics and insurance policies. Local minutes, ecclesial record books, ecclesial rule books, letters and other surviving documents fully support the picture as presented by Norrie. This picture of diversity was not reflected, for example, in the 58 chapters on this period of Christadelphian history written by Robert Roberts in Life Dr. T., and MDAMW. The Christadelphian past tended, here, to be written rather in the manner of the Whig historians.”
The phrase “Baptised Believers” was a widely used phrase by those who left various churches to follow the teachings of John Thomas (to varying degrees) and before the name Christadelphian was coined in 1865 and generally adopted.
The book, “The Early History of the Kingdom of God in Britain” is available on this site. To read some early Christadelphian thoughts on the increasing requirements of doctrinal conformity there is an early article on “High Church Christadelphians”.