"Any system of law requires certain basics to make it operative. One is a penalty for violations. Another, equally essential, is the inviolability* of an oath. It is utterly impossible to adminster justice, either in the punishment of wickedness and vice or the adjudication of civil disputes, unless men can be required to tell the truth on pain of heavy penalty. The requirement is met by an appeal to what men regard as sacred and inviolable. For Christians, it means an appeal to God. To appeal is to call upon. To call upon the name of God is to take a solemn oath or to swear. To do that in anything but the utmost adherence to the full truth is to split asunder the whole structure of administration of law in all its forms."
--T. Robert Ingram, from The World Under God's Law (St. Thomas Press, 1962, p.43)
* Secure from violation or profanation; unbreakability