From another version of this book I know the clause in boldface can be reworded as "It's certain". I searched the phase "past doubt" in Google but few related results were returned. So I wonder if this usage is a very old-fashioned.
In a word, however useful medicine well administered may be to us who live in a state of society, it is still past doubt, that if, on the one hand, the sick savage destitute of help, has nothing to hope from nature, on the other, he has nothing to fear but from his disease; a circumstance, which often renders his situation preferable to ours.
-- from Rousseau's “Discourse on Inequality”