My primary school teacher used to swear by Jove to emphasise how impressed he was when one of his pupils got full marks on a spelling test. At the time, I didn’t find this odd. And at the time also I had no idea that in some countries swearing by a deity, or even talking lightly about a prophet could result in a government-imposed death sentence, or alternatively being lynched by a mob or beheaded, stabbed or shot by any random follower of a merciful deity with an unusual definition of mercy.