Explanation

Believing is a gerund used as a noun.

The gerund phrase functions as the subject of the sentence.

gerunds by adding “-ing” to a verb base, for instance, “seeing” or “believing,” as in “seeing is believing.”

The underlying sense of “seeing” and “believing” in that sentence is “the act of seeing” and “the act of believing.”

Note that “-ing” can turn an English verb base into a gerund (a noun)

ND-18-8-2023