Esders has always been fascinated by this voluntary form of binding oneself, he explains. When reading texts from the ancient and medieval worlds, he was struck by how very important the act of swearing and the exact wording of oaths were. “Wherever we find oaths, we run into situations where firm rules have to be set for things because competing norms and role expectations are in conflict.” Every oath, he says, is sure to lead historians to a highly sensitive point in past societies – and, at the same time, to situations where an oath drove people into severe moral dilemmas.

The Roman Empire adopted Christianity in late antiquity. Before that, vows to the gods had been customary, and every Roman soldier also swore an oath of allegiance. But the new national religion represented a break with this tradition. After all, in the Sermon on the Mount, Jesus had forbidden his followers to swear. “It took great effort, but people forced themselves to allow oaths under certain conditions – evidently because things wouldn’t work without them,” Esders explains.

The situation was amplified around 500 A.D., when the western Roman Empire crumbled. The power of the state dwindled, and society permitted a loosening of fixed rules. This created a lack of binding legal rules, so new contractual bonds were needed. The concept of oaths flourished after that. “Many things were newly negotiated at that point and safeguarded through oaths,” Esders says.

But what is an oath, exactly? What happens during the process of swearing? “The first thing is that a deity is called on to support a person’s own promise or statement,” Esders explains. By referring to a higher power far beyond the individual, the person swearing an oath not only emphasizes the depth of his or her intentions, but also accepts whatever sanctions may be imposed by that power in the event of perjury or oath breaking.

In the worst case, he says, people are even risking their life after death. The historical record contains references to oaths of loyalty and allegiance from Byzantium and early Islam, and the Romans also swore oaths of allegiance, as described above, and other oaths. In late antiquity, Christians in some places even swore oaths inside Jewish synagogues because oaths sworn there were believed to be more binding and to confer better protection. The act of swearing oaths has always taken place, across all of the world’s religions and regions.

“In some societies, oaths were sworn not only to a specific deity, but also on the lives of a person’s own children, or even on one’s own testicles,” the historian explains, “basically on anything that is sacred or very important.” What all oaths had in common was that they were given in public, so they were a way of making a pledge in front of an audience. The more witnesses there were, the lower the chances that someone would try to escape his or her oath.

Breaking an oath was practically impossible to justify to others. In this way, oaths also became a kind of “social cement”: Anyone who was under oath merited greater trust and confidence, since the fact of calling on a higher power gave the sworn statement more weight, and perjury was subject to sterner punishment. To this day, stiff penalties are imposed on anyone who lies in court.

The fact that the New Testament prohibits oaths, while at the same time there has never been an era fraught with as many oaths as the Christian Middle Ages, is an interesting paradox, Esders says. In medieval society in particular, the importance of swearing oaths and vows is impossible to overlook: Kings swore an oath at coronation that they would rule justly and with the people’s interests in mind. In return, their subjects swore to be loyal to them, to make war for them, and to follow their orders.

The legal systems of medieval cities and territories were based on an oath that was renewed each year. And ultimately, the entire feudal system would have been impossible without the oath sworn by a vassal to his liege lord. In short, oaths were always what held medieval society together; they affirmed the necessary trust as a foundation of the society in question. “For a long time, even guilds and universities themselves were actually conspiracies, in a way – they were sworn societies of equals,” Esders says. “At medieval universities, students had to promise their professors under oath that they would not attempt to take revenge on them after exams.”

Although oaths have lost a great deal of their importance, they are still sworn at crucial moments, such as when governments and Presidents are sworn in and in court. The original meaning of oaths has carried through to the present day in this practice: the idea of undertaking a voluntary obligation before God or fate, emphasizing one’s intentions beyond the usual measure. “Oaths still show us a great deal today and tell us something about the person swearing them, such as whether that person uses a phrase like ‘so help me God,’” Esders says.

And, of course, oaths are still an important subject of relations between religions. Some ten years ago, when Keith Ellison, the first Muslim to serve in the U.S. Congress, gave his oath of office, he placed his hand on a copy of the Koran that had once belonged to Thomas Jefferson – placing himself in a longstanding republican tradition promoting religious freedom. It would be interesting to see, whether he would also find a copy of the Koran in the library of Donald Trump's family as well.