The letters said: "…. in a momentous drama giving the answer to the great problem: Should a woman tell?"
We can’t make snap judgments and reach momentous decisions on the spur of the moment, can we?
That’s not the way to be moral-" Rearden started, and stopped abruptly, realizing that— it was the way, the only way left, realizing through how many twists of intellectual corruption upon corruption this boy had to struggle toward his momentous discovery.
There are no more uses of "momentous" in the book.