The question, especially at the beginning, sounds a bit odd in its meaning.
The (i) _________ of “surds"—
irrational roots—with the Pythagoreans’ faith that all phenomena in the universe could be
expressed through harmonious ratios of whole numbers led the cult to (ii)_________ any mention of their existence to the uninitiated.
if we take a look to the first words - irrational - that is the hint to have B in the first blank.
As a contrast, the second part of the sentence tells us that the universe could be explained by something is harmonious - numbers. However, this numbers are fully understood by a restrict number of people or scientists which turns out is like a cult. This cult proibits to the people external to it to know even the existence of them
That is the meaning and unfolding of the sentence itself.
Regards
_________________