Choose to believe

Hello Rabbi,
The Gemara in Nidda 6 says, following the incident of Gedaliah ben Ahikam, “The one who has not accepted the word, even though he has accepted it, does not express it.” Following this Gemara, the Chofetz Chaim states in his book that a person is prohibited from accepting slander, meaning believing it, but only from fearing it, and if the person believes it, then he is committing the offense of accepting slander.
Ostensibly, this means that faith is a matter of choice and not of fact, as the rabbi claims.
hand.