First, some definitions:
- As the dictionary linked by cmw says, "the rather" can mean "the more so; especially."
- "Forasmuch as" can mean "because of the fact that" or simply "because."
In the passage, the Woodman (Fate) and the Farmer (Death) are personifications of the historical processes that led, imperceptibly but inevitably, to the French Revolution. This sentence gives two reasons why no one noticed ("heard") them until it was too late:
- First, no one heard them because they "work silently" and "with muffled tread." The processes that lead to a revolution are not obvious to people before the revolution happens.
- But second, and even more importantly ("the rather"), no one heard them because no one wanted to hear them. Anyone who said, "I think France may be heading toward a revolution," would be condemned by those in power as an atheist and a traitor.1 They would be considered a traitor, because someone who suggested that it was possible to overthrow the government might actually want to overthrow the government. And they would be considered an atheist, because France's monarchy was supposedly ordained by God;2 anyone who claimed that it could be overthrown must believe that the God who supported it did not exist.3
Thus, Dickens is saying that the French ignorance about the oncoming revolution was largely a willful ignorance. It is true that the Woodman and the Farmer were hard to notice. But furthermore, anyone who did notice them would be branded an atheist or a traitor. No one wanted that, and so everyone went on pretending that everything was fine--until it wasn't.
1 Both of which accusations would probably have led to horrific punishments. Compare the case, earlier in the paragraph, of the youth who was tortured to death because he failed to kneel down to honor some monks.
2 Note the reference near the end of the chapter to "divine right(s)," the European belief that kings and queens ruled because God gave them the right to rule.
3 Of course, a third possibility is that God does exist but the French monarchy was not ordained by God. If this is the view that Dickens himself held, he may be poking fun at people who believe in divine right by implying that they cannot distinguish between believing that monarchies are God-ordained and believing in God at all.