Taking part in a live phone-in on LBC, the Lib Dem leader took a question from Brian, who wanted the politician to clarify his stance on homosexuality.
The pro-EU candidate refused to answer whether he thought it was a sin but insisted he was a champion of LGBT rights.
But the caller blasted back: “I'm not asking you about that, what I want to do is find out more about you as a human being, as a person and your religion is a very important part of you as a person while you are standing for this election.
“I want to know why you think homosexuality is a sin.”
Mr Farron replied: “I am a political leader and I am a liberal to my fingertips.”
Tim Farron refused to answer if he thought homosexuality was a sin
You know what, we’re not going there. My personal faith is my personal faith
Host Nick Ferrari then interrupted and probed the politician further.
“Is it a sin to be homosexual,” he asked again.
But Mr Farron said: “I dealt with that weeks ago.”
Ferrari repeatedly asked him to remind listeners of his stance to no avail.
“You know what, we’re not going there. My personal faith is my personal faith,” said Mr Farron.
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He was then prompted a further four times, but the politician refused to budge and said the caller had “heard me talk about it some weeks ago”.
When asked why he would not answer the question, Mr Farron replied: “Because to be honest with you, a person who is the leader of a political party, it’s their job and [as] someone who’s passionate about LGBT+ rights, prove it by your actions not by your words.
“And my actions absolutely are 100 per cent are about defending LGBT+ rights and as a party, you look at what we’ve done, Lynne Featherstone, my colleague in government, she introduced equal marriage, I am proud of what she’s done.”
When asked a final time, he insisted: “I’ve already answered that, let’s move on.”
In April, Mr Farron told the station being gay was “not a sin”.
He said: “I have been absolutely clear this morning, in the House of Commons… being gay is not a sin.”