I would however like to point out the error of successionist history. Baptists who cling to the "trail of blood" and other heritage are sorely mistaken... All organizations, even the catholic/orthodox church, at one point had much truth. The fact is, God always maintains a remant, but He doesn't need human organizations to do it!
So while I agree with these successionist historians that there were always baptists throughout the course of history, they weren't necessarily called baptistst/donatists/etc... but in fact were individuals and we won't always see a clear "line" of baptist heritage. But no doubt, I believe there have always been true Christians since the time of Christ - and even those that believed in baptizing believers only and held to baptist distincitives, although it's highly likely they were murdered as soon as their beliefs were discovered. I would also suspect these individuals were church officials who had access to the Scriptures.