"The openness of Christadelphianism in its early days permitted brethren, once converted, to stay nominally within their churches of origin, causing a wider spread of the new views amongst those with some sympathy for them than would have occurred under tighter restrictions regulating communion."
In its early days Christadelphianism was a movement rather than a denomination, with its roots in the Believers Movement (1848-1864) and the Restoration Movement (1800 to today) before that. Christadelphians maintained their contacts with other Christians and visited other churches, or remained within them. While later generations outlawed marriage with non-Christadelphian "unbelievers", John Thomas encouraged Christadelphians to marry "amiable, well-disposed, tractable, and God-fearing" people and wrote quite sternly against those elders who wanted to make it a rule (and a condition of fellowship) that Christadelphians could only marry Christadelphians. ("Is it lawful to marry unbelievers?" The Ambassador of the Coming Age 1866 pages 91 to 97).
Consequently there seems to have a been a healthy interchange of ideas between the Believers and early Christadelphians and other Christians. John Thomas's own writings reveal that he read widely and readily borrowed ideas from commentators and Bible scholars, regardless of their denomination. He was especially influenced by Bible scholars in the Restoration Movement and groups which sprang out of it (including groups we now know as the Disciples or Churches of Christ, the Adventists, the Church of God, the Christian Connection, the Bible Students, and others). In turn, Christadelphian speakers and writers seem to have had an influence on some people in these groups, and others. As a movement within the wider Body of Christ early Christadelphians were able to share what they had learned, while continuing to modify their own views. For a time the interchange was positive, productive and healthy.
Point # 3 in my summary of what Christadelphians set out to be is this:
Christadelphians originally saw their place within the Body of Christ, and sought both to learn and to influence as they grew in grace and knowledge. They recognised and appreciated the valuable contribution of other Christians and would also endeavour to share their own perspectives in the hope that the whole Body of Christ would come to maturity.