Pastor James McConnell, a prominent Belfast preacher, is in trouble for calling Islam "satanic". This has been condemned as hate speech by Amnesty, although it doesn't seem more hostile than many of the atheist attacks on Muslim beliefs from people who would despise McConnell as an ignorant hate-monger. But McConnell's language is much more inventive. What is it about Reformed Protestantism that gives it such a tradition of invective?

Part of this stems from its founding controversies. The classic statement of English-speaking Calvinism, such as McConnell represents, is the Westminster Confession, drawn up in 1647 in the aftermath of the English civil war. It is not distinguished by its tolerance.

Degenerate churches (generally understood to be Roman Catholic) were described as "synagogues of satan". Mixed marriages – those involving "infidels, papists, and other idolaters" or those who maintain "damnable heresies" – are condemned.

Monastic vows of poverty, chastity, and obedience are dismissed as "superstitious and sinful snares, in which no Christian may entangle himself". So much for Mother Teresa, who would have taken those vows. She is supposed to be damned in hell at this moment and forever: the belief that anyone outside a Calvinist church could be saved from damnation is "very pernicious and to be detested".

These are still among the official statements of faith of the Church of Scotland, and thus, I suppose, still believed in some remote sense by the Queen when she is north of the Border and becomes a member of that church.

The Westminster Confession is very mild indeed by the standards of Luther's invective, or even of that time's standards of literary criticism – "Cibber! Write all your verses upon glasses / The only way to save 'em from our arses", as Alexander Pope wrote about one rival.

But it's also important to remember that religious anathemata were thought to describe objective facts. To say that something was "damnable" was not merely an expression of preference but a statement of something true and important about the moral order of the universe, much as we now use "racist".

Quite probably moral language has to work like that. It needs to express emotion as well as to describe the world, and in religious terms cursing goes back at least as far as blessing, and curse tablets are among the earliest forms of religious writing that we have.

Perhaps the most spectacular curse in any British language was that pronounced in 1525 by the Archbishop of Glasgow, Gavin Dunbar, on the Border reivers, which ends (in translation, for it was written in Scots): "I condemn them perpetually to the deep pit of hell, to remain with Lucifer and all his kind, and their bodies to the gallows of the town hill, first to be hanged, then rent and torn with dogs and swine and other wild beasts, an abomination to all the world. And their candles go from your sight, just as their souls go from the vision of God and their good fame from the world, until they forbear their open sins, before mentioned, and arise from this terrible cursing and make satisfaction and penance."

This is the culture in which Calvinist doctrines took root in Scotland, and from which they were carried to Ulster. I'm sure McConnell would envy such eloquence – at least until he reflects that Dunbar was a Roman Catholic Archbishop, and thus a detestable heretic.