The Liberal Democrat leader was repeatedly asked about his attitudes towards homosexuality during LBC's Leaders Live with Nick Ferrari.
A caller named Brian said there was a "huge question mark" over Mr Farron's views, accusing the leader of thinking "homosexuality is a sin".
Mr Farron said he was "a Liberal and ever since my day of joining the party 31 years ago I've been an active campaigner for LGBT+ rights".
The caller asked Mr Farron to clarify his views on homosexuality, to which he replied: "I'm talking to you about my passionate concern about LGBT+ rights."
But the caller said the leader's religion was "very important", adding: "I want to know why you think homosexuality is a sin, and why you think that is a crime."
Mr Farron said: "My view very, very much is about I am a political leader and I am a Liberal to my fingertips," adding that he "dealt" with the issue "weeks ago".
The Lib Dem leader has previously said he did not think gay sex was a sin after being dogged with questions about his beliefs.
When asked to remind the caller about his views, he said: "We're not going there - my personal faith is my personal faith.
"I dealt with that weeks ago."
He said the caller had "heard me talking about it some weeks ago".
Ferrari asked why Mr Farron would not remind listeners, to which he replied: "To be honest with you, a person who is a leader of a political party, it is their job - and someone who is passionate about LGBT+ rights - to prove it by your actions and not by your words, and my actions absolutely are 100% about defending LGBT+ rights."
The presenter said Mr Farron would not answer whether it was sin, and he replied: "I've already answered that - let's move on."
