At the end of his presidency, George Washington published a letter reflecting on the character of the nascent American republic. Later known as his Farewell Address, the letter famously warned against the dangers of domestic political parties and entangling foreign alliances. In addition, Washington extolled the foundations of a virtuous citizenry: “Of all the dispositions and habits, which lead to political prosperity,” he proclaimed, “Religion and morality are indispensable supports.” Washington then offered an example: “Let it simply be asked where is the security for property, for reputation, for life, if the sense of religious obligation desert the oaths, which are the instruments of investigation in Courts of Justice?” For his eighteenth-century audience, Washington’s reference to the religious content of oaths was straightforward. Testimony under oath could be trusted because witnesses put their souls on the line. As William Blackstone explained in his Commentaries on the Laws of England, “The belief in a future state of rewards and punishments, the entertaining just ideas of moral attributes of the supreme Being, and a firm persuasion that he superintends and will finally compensate every action in human life ... are the ground foundations of all judicial oaths.” Consequently, prospective witnesses who did not believe in divine punishment were barred from testifying—or, in legal parlance, declared “incompetent” to testify. [..]
Jud Campbell, Testimonial Exclusions and Religious Freedom in Early America, 37 Law & History Review 431 (2019).