Judging by his rhetoric, Cardinal Keith O'Brien, the leader of the Catholic Church in Scotland, is not a man seeking converts.

Challenged on the Today programme this morning over his comparison of gay marriage with slavery in a column for the Sunday Telegraph, O'Brien insisted that it was "a perfectly good example of what could happen in our own country".

He wrote in the paper:

Imagine for a moment that the Government had decided to legalise slavery but assured us that "no one will be forced to keep a slave". Would such worthless assurances calm our fury? Would they justify dismantling a fundamental human right? Or would they simply amount to weasel words masking a great wrong?

Here's the full, shocking exchange with John Humphrys.

John Humphrys: The idea of introducing the notion of something as grotesque as slavery, and I use the word there advisedly, the ultimate denial of a person's human rights, in this context will itself prompt many people to think that this is grotesque, that your notion of it is grotesque.

Keith O'Brien: I'm not saying that it's grotesque, perhaps to some people it might appear grotesque.

Humphrys: But you're a Cardinal! Should you be using that kind of language? Equating, even as remotely as this, the notion of gay relationships with slavery?

O'Brien: I think it's a very, very good example as to what might happen in our own country at this present time and I feel I've duty, a responsibility to preach and to teach and this is one of the ways in which I do it.

Humphrys: Sorry, you lost me a little here, what might happen? Legalising slavery and the equivalent of homosexual marriage?

O'Brien: It is a perfectly good example as to what could happen in our own country if we go this way. I'm simply handing on the teaching of the Christian Church down through the years.