The invocation of God's name to bear witness to the truth. A person, being conscious of his or her own fallibility, professes by an oath that God is omniscient and the omnipotent avenger of falsehood. For an oath to be licit, the statement sworn to must be true; there must be sufficient reason for swearing, i.e., regarding some matter of importance or because the circumstances demand an oath, as in a court of law; and the statement itself must not be sinful, e.g., not disclosing a secret that should not be revealed.
Oaths are assertive when God is invoked as witness to the truth of a past or present event, e.g., that a crime was not committed. They are promissory when God is invoked to bear witness not only to a future act but also to a person's present intention of doing or omitting something, e.g., the promise to fulfill the duties of one's office.
Oaths may also be distinguished as invocatory and imprecatory. They are invocatory when God is simply called upon as witness to the truth; they are imprecatory when, in addition, he is also invoked as the avenger of falsehood. In the Old Testament oaths of imprecation were very frequent, e.g., in the expression, "The Lord do so to me and more also." In current usage they now occur in the familiar form, "So help me God." (Etym. Greek oetos, a going; fate.)