Looking at a dictionary, I found that "one and all" means "everyone", but in the excerpt below from Schopenhauer's On Human Nature, it doesn't seem to make much sense. Also note that the verb maintain doesn't have an s, which seems to imply that "one and all" isn't used to refer to "everyone".
Excerpt:
To say that the world has only a physical and not a moral significance is the greatest and most pernicious of all errors, the fundamental blunder, the real perversity of mind and temper; and, at bottom, it is doubtless the tendency which faith personifies as Anti-Christ. Nevertheless, in spite of all religions—and they are systems which one and all maintain the opposite, and seek to establish it in their mythical way—this fundamental error never becomes quite extinct, but raises its head from time to time afresh, until universal indignation compels it to hide itself once more.