This kind of construction is called a double negative, and as a result it expresses an affirmative.
Literal
This-kind-of form [object-を] double-negative [quotative-と] say [but-が], result [as-として] affirmative [object-を] express [nominalizer-こと][result-と] become.
二重否定 'double negative' and 肯定 'affirmative' are linguistic terms. こととなります is a formal version of ことになる 'it turns out to be the case that.' The sentence explains a metalinguistic concept — how double negatives produce affirmatives — in textbook register.