I COINED a new internet axiom this week, and immodestly named it Vallely's Corollary. It is an adjunct to Godwin's Law, which declares that "as an online discussion grows longer, the probability of a comparison involving Nazis or Hitler increases." When such a comparison is made, the precept continues, the debate is over; and whoever mentioned the 20th century's nonpareil nasty is deemed to have lost through feebleness of argument.

Vallely's Corollary arose from the debates about Pope Francis's new eco-encyclical, Laudato Si'. For Hitler, it substitutes Galileo - the last refuge of any desperate rationalist intent on asserting that the Roman Catholic Church knows nothing about science. A scholastic might name this reductio ad galileum.

It is sometimes difficult to work out whether so-called rationalist contempt for religion proceeds from ignorance or arrogance. But there is also the possibility of duplicitous malice. The recent attacks on Tim Farron, one of the two politicians vying to be leader of the Liberal Democrats, offer an interesting case study.

Two aides working for his rival, Norman Lamb, have resigned, after party members received calls which appeared to be soliciting information about the two candidates' stances on issues such as abortion and gay marriage. The intention of the callers was actually to raise doubts in electors' minds about Mr Farron's Christian beliefs. Mr Lamb apologised to Mr Farron, but the innuendos continued in political circles with questions like "Can a Christian lead a liberal party?"

This is a patently absurd question, since a previous leader, Charles Kennedy, was a practising Roman Catholic. But that has not stopped continuing questions about the supposed incompatibility between Christianity and liberal values.

"One hopes for better among the Liberal Democrats, but of course I'm not surprised," Mr Farron told one interviewer. "In elections, you can choose to play the man or the ball, and if you choose to play the man, you look for a perceived political weakness - and some people perceive my faith as a weakness. In the US, everyone has to invent a faith to get elected. Here, you're not allowed to have one."

That disparity was underscored this week by the remarkable eulogy that President Obama delivered for the nine people shot dead in Charleston. To us an alien blend of politics and religion, with the President ending by breaking into an unaccompanied version of "Amazing Grace", it was both powerful and moving.

Mr Farron, by contrast, has continued to be subjected to weasel-worded suggestions that his "religious fervour" will "trump his political principles". Critics have, for example, claimed the incompatibility of his declaring that every abortion is a tragedy while upholding the legal right of women to have a termination if they so decide. Faux gasps of outrage greeted that, though it is no more self-contradictory than being against adultery but not wanting to make it a criminal offence.

Mr Farron has been sanguine in his response, saying rather alarmingly that "I expect to be misconstrued", and asserting that the question "Can a Christian lead a liberal party?" "misunderstands liberalism". It also misunderstands Christianity. Perhaps out of ignorance. Perhaps wilfully.