Writing for i, Mr Farron said politicians with deeply held faith should not vote for laws to make people who are not Christian “live as though they are”.
But he said it was “illiberal” not to allow Ms Forbes to express her views in public, even if others disagreed with her.
After serving as Lib Dem leader for two years, Mr Farron – who had voted in favour of same sex marriage in parliament – stepped down in 2017 saying he had become “torn between living as a faithful Christian and serving as a political leader”.
Ms Forbes’ pitch to succeed Nicola Sturgeon ran into difficulty earlier this week when she said she would not have voted for gay marriage if she had been an MSP in 2014 and said it was “wrong” for children to be born outside marriage.
In his comment piece, Mr Farron said: “As a Christian, I’m not offended by the stick we sometimes get – after all, Jesus promises that we will have trouble, and our response must be to turn the other cheek and show love and gentleness. But I am offended by all this as a liberal.
“We can’t have a position where people with ‘non-religious’ world views are permitted to take their value system into the public square but people of faith are not. That would be both illiberal and stupid.
“However, I do firmly believe it’s not our responsibility to legislate to make people who are not Christians live as though they are. That would also be illiberal. In the House of Commons I supported same-sex marriage, for instance.”
The MP for Westmorland and Lonsdale added: “What is unfair is when Christians who hold particular beliefs are instantly hounded into a box labelled ‘the enemy’. We can’t claim to live in a liberal, tolerant society if we instantly dismiss faith-fuelled views as bigoted and outdated.
“The culture war has created a society where the knee-jerk response to hearing a jarring world view is hostility.
“We’d be better liberals, and better people, if we replaced that hostility with curiosity instead. Why aren’t we more interested in knowing why people think what they do?”
During the 2017 general election campaign, Mr Farron faced questions over whether he believed gay sex was a sin.
In his resignation speech, Mr Farron – who had voted in the Commons in favour of same sex marriage – said: “To be a leader, particularly of a progressive liberal party in 2017 and to live as a committed Christian and to hold faithful to the Bible’s teaching has felt impossible for me.”