It means something like "preferably". During the years leading up to the French Revolution, the peasants and the poor urban dwellers, who were instrumental in this, were obliged to go about their work quietly and discreetly, pretending that everything was normal, because if others had even realised what what their ultimate goals were (bringing down the monarchy and dismantling the the power structures of the aristocracy) and made this known, even they would have been considered as traitors and atheists. (The Roman Catholic church was all-powerful in France at this time). So, rather than let anybody know what they were up to, the Woodmen and the Farmers, (symbolising fate and death respectively) thought it preferable to do their revolutionary work in secret.

But, that Woodman and that Farmer, though they work unceasingly, work silently, and no one heard them as they went about with muffled tread: the rather, forasmuch as to entertain any suspicion that they were awake, was to be atheistical and traitorous.