The calling upon God to witness the truth of a statement.
As an appeal to the testimony of God, it is an act of religion.
An oath may be:
To have a valid oath there must be the intention, at least virtual, of invoking the testimony of God, and a word or sign by which such an intention is manifested.
Such a word or sign is contained in the formulas, "God is my witness," "I swear by God," "May God destroy me if," etc.
Other expressions such as "By my conscience" or "As God lives" are; at most doubtful.
To make an oath lawful it is necessary that what one swears be true or at least beyond a prudent doubt or, in the case of a promissory oath, be sincerely intended and not unlawful.
Moreover there must be a sufficient reason for taking an oath.
Such sufficient reason is had when an oath is ordered by lawful authority or required for God's honor or our own or our neighbor's good.
- assertory, whereby God is called to witness the truth of an assertion of fact, past or present
- promissory, whereby God is called upon as a witness to a resolution, a vow, or an agreement made with another party, and as a "guaranty and pledge of future fulfillment
New Catholic Dictionary