Which brings us to the question how does an oath work? In most of modern life, we have drained much of the meaning out of the few oaths that we still take, in part because we tend to be very secular and so don’t regularly consider the religious aspects of the oaths – even for people who are themselves religious. Consider it this way: when someone lies in court on a TV show, we think, “ooh, he’s going to get in trouble with the law for perjury”. We do not generally think, “Ah yes, this man’s soul will burn in hell for all eternity, for he has (literally!) damned himself.” But that is the theological implication of a broken oath!

So when thinking about oaths, we want to think about them the way people in the past did: as things that work – that is they do something. In particular, we should understand these oaths as effective – by which I mean that the oath itself actually does something more than just the words alone. They trigger some actual, functional supernatural mechanisms. In essence, we want to treat these oaths as real in order to understand them.

So what is an oath? To borrow Richard Janko’s (The Iliad: A Commentary (1992), in turn quoted by Sommerstein [in Horkos: The Oath in Greek Society (2007)]) formulation, “to take an oath is in effect to invoke powers greater than oneself to uphold the truth of a declaration, by putting a curse upon oneself if it is false”. Following Sommerstein, an oath has three key components:

First: A declaration, which may be either something about the present or past or a promise for the future.

Second: The specific powers greater than oneself who are invoked as witnesses and who will enforce the penalty if the oath is false. In Christian oaths, this is typically God, although it can also include saints. For the Greeks, Zeus Horkios (Zeus the Oath-Keeper) is the most common witness for oaths. This is almost never omitted, even when it is obvious.

Third: A curse, by the swearers, called down on themselves, should they be false. This third part is often omitted or left implied, where the cultural context makes it clear what the curse ought to be. Particularly, in Christian contexts, the curse is theologically obvious (damnation, delivered at judgment) and so is often omitted.

While some of these components (especially the last) may be implied in the form of an oath, all three are necessary for the oath to be effective – that is, for the oath to work.

A fantastic example of the basic formula comes from Anglo-Saxon Chronicles (656 – that’s a section, not a date), where the promise in question is the construction of a new monastery, which runs thusly (Anne Savage’s translation):

These are the witnesses that were there, who signed on Christ’s cross with their fingers and agreed with their tongues … “I, king Wulfhere, with these king’s eorls, war-leaders and thanes, witness of my gift, before archbishop Deusdedit, confirm with Christ’s cross” … they laid God’s curse, and the curse of all the saints and all God’s people on anyone who undid anything of what was done, so be it, say we all. Amen.” [Emphasis mine]

So we have the promise (building a monastery and respecting the donation of land to it), the specific power invoked as witness, both by name and through the connection to a specific object (the cross – I’ve omitted the oaths of all of Wulfhere’s subordinates, but each and every one of them assented “with Christ’s cross”, which they are touching) and then the curse to be laid on anyone who should break the oath.

Of the Medieval oaths I’ve seen, this one is somewhat odd in that the penalty is spelled out. That’s much more common in ancient oaths where the range of possible penalties and curses was much wider. The Dikask‘s oath (the oath sworn by Athenian jurors), as reconstructed by Max Frankel, also provides an example of the whole formula from the ancient world:

I will vote according to the laws and the votes of the Demos of the Athenians and the Council of the Five Hundred … I swear these things by Zeus, Apollo and Demeter, and may I have many good things if I swear well, but destruction for me and my family if I forswear.

Again, each of the three working components are clear: the promise being made (to judge fairly – I have shortened this part, it goes on a bit), the enforcing entity (Zeus, Apollo and Demeter) and the penalty for forswearing (in this case, a curse of destruction). The penalty here is appropriately ruinous, given that the jurors have themselves the power to ruin others (they might be judging cases with very serious crimes, after all).

Bret Devereaux, “Collections: Oaths! How do they Work?”, A Collection of Unmitigated Pedantry, 2019-06-28.