Kevin Rudd has insisted his views in favour of same-sex marriage are compatible with his Christian faith, saying a literal interpretation of the bible that opposed gay unions would also dictate support for slavery.

Challenged by a pastor and Christian radio announcer on the ABC’s Q&A program on Monday night as to why as a Christian he did not follow the teaching of the bible that marriage was between a man and a woman, Rudd replied: “Well, mate, if I was going to have that view, the bible also says that slavery is a natural condition because St Paul said in the new testament: ‘slaves be obedient to your masters,’ and therefore we should all have fought for the confederacy in the US civil war.”

He added: “I mean, for goodness sake, the human condition and social conditions change. What is the central principle of the new testament? It is one of universal love, loving your fellow man,” Rudd said, during a passionate performance in which he insisted he was still not contemplating losing Saturday’s election despite Labor’s declining standing in the opinion polls.

Rudd seemed to find his campaigning mojo on Sunday at Labor’s launch in Brisbane, where he crystallised his pitch to voters into a choice between Labor’s commitment to protect jobs and the allegation that the Coalition intended cuts to services, and a plea to voters not to vote for Tony Abbott if they still harboured doubts about the Coalition leader’s capacity to lead the nation.

Rudd changed his stance on gay marriage before returning to the Labor leadership this year, publishing an essay to explain his views. He has promised to re-submit legislation legalising same sex marriage within 100 days if Labor is re-elected.

Abbott believes marriage is between a man and a woman and has said it would be up to the Liberal party room to decide whether, unlike the last vote on the issue, Liberal MPs would allowed a conscience vote on the issue.

“I do not believe people choose their sexuality; they are gay if they are born gay,” Rudd said during Monday night’s program. “It is how people are built, and therefore the idea that this is somehow an abnormal condition is wrong … If you accept it is natural and normal to be gay, then it follows from that … people should not be denied the opportunity for legal recognition ... of their relationship.”

Rudd said his greatest mistake as prime minister was deferring the original emissions trading scheme, and held up the stimulus package at the time of the global financial crisis as his greatest achievement.

But he refused to entertain a question about what he would do if he lost on Saturday, saying he was “not going to engage in hypothetical land” because he was “not in that frame of mind”.

In implicit criticisms of Julia Gillard’s time as leader he said he had never liked the idea of “class rhetoric” and had always supported a floating carbon price rather than a fixed carbon tax, even though his original emissions trading scheme also began with a one-year fixed price.

And Rudd appeared to tone down his attack of the coverage of the election campaign by News Corp papers, saying it was obvious Rupert Murdoch preferred Abbott as prime minister but he had “great confidence that the Australian people make up their own mind, they know whether the reporting is balanced and fair, they will work it out for themselves are both sides of politics being put under real scrutiny.”

He said he believed “freedom of the media goes to a lifeblood of a democracy”, but rejected the idea of further media regulation, saying that “you have to be very careful about getting the government directly involved in the regulation of the media, because where does the slippery slope stop, I am not quite sure.”