Kitto's Popular Cyclopedia of Biblial Literature
Oath, an appeal to God in attestation of the truth of what you say, or in confirmation of what you promise or undertake. Cicero correctly terms an oath a religious affirmation; that is, an affirmation with a religious sanction. Hence it appears that there are two essential elements in an oath: first, the human, a declared intention of speaking the truth, or performing the action in a given case; secondly, the divine, an appeal to God, as a Being who knows all things and will punish guilt. According to usage, however, there is a third element in the idea which 'oath' commonly conveys, namely, that the oath is taken only on solemn, or, more specifically, on juridical occasions.
The essence of an oath lies obviously in the appeal which is thereby made to God, or to divine knowledge and power. The customary form establishes this, 'So help me God.' The Latin words (known to have been used as early as the sixth century), whence our English form is taken, may be thus rendered: so may God and these holy gospels help me; that is, 'as I say the truth.' The present custom of kissing a book containing the Gospels has in England taken place of the latter clause in the Latin formula.
Oaths did not take their origin in any divine command. They were a part of that consuetudinary law which Moses found prevalent, and was bound to respect, since no small portion of the force of law lies in custom, and a legislator can neither abrogate nor institute a binding law of his own mere will. Accordingly, Moses made use of the sanction which an oath gave, but in that general manner, and apart from minute directions and express words of approval; which shows that he merely used, without intending to sanction, an instrument that he found in existence and could not safely dispense with. Examples are found in , where an oath is ordered to be applied in the case of lost property; and here we first meet with what may strictly be called a judicial oath ().
The forms of adjuration found in the Scriptures are numerous. Saul sware unto Jonathan, 'As the Lord liveth' (). 'A heap and a pillar' were for a witness between Laban and Jacob, with the ensuing for a sanction, 'The God of Abraham and the God of Nahor, the God of their father, judge betwixt us. And Jacob sware by the fear of his father Isaac' (, sq.). A common formula is, 'The Lord do so to me and more also' (; ), which approaches nearly to our modern form, 'So help me God,' and is obviously elliptical. Reference appears to be had to the ancient custom of slaying some animal in confirmation of a treaty or agreement. The animal thus slain and offered in a burnt offering to God became an image or type, betokening the fate which would attend that one of the two contracting parties who failed in his engagement; subsequently the sacrifice was in ordinary cases omitted, and the form came in itself to have the force of a solemn asseveration.
An oath, making an appeal to the divine justice and power, is a recognition of the divinity of the being to whom the appeal is made. Hence to swear by an idol is to be convicted of idolatry. Such an act is accordingly given in Scripture as a proof of idolatry and a reason for condign punishment. 'How shall I pardon thee for this? Thy children have forsaken me, and sworn by them that are no gods' (;;; ).
Other beings besides God are sometimes added in the form of an oath: Elijah said to Elisha, 'As the Lord liveth, and as thy soul liveth' (; ). The party addressed is frequently sworn by, especially if a prince: 'As thy soul liveth, my lord, I am the woman,' etc. (;;; ). The Hebrews as well as the Egyptians swore also by the head or the life of an absent as well as a present prince: 'By the life of Pharaoh' (). Hanway says that the most sacred oath among the Persians is 'by the king's head.'
The oath taker swore sometimes by his own head (); or by some precious part of his body, as the eyes; sometimes, but only in the case of the later Jews, by the earth, the heaven, and the sun (); as well as by angels; by the temple (), and even by parts of the temple (). They also swore by Jerusalem, as the holy city (). The Rabbinical writers indulge in much prolixity on the subject of oaths, entering into nice distinctions, and showing themselves exquisite casuists.
We have already intimated that it was usual to put the hand under the thigh (; ). The more usual employment of the hand was to raise it towards heaven; designed, probably, to excite attention, to point out the oath-taker, and to give solemnity to the act (). In the strongly anthropomorphitic language of parts of the Scripture, even God is introduced saying, 'I lift up my hand to heaven, and say, I live forever' (). It can only be by the employment of a similar license that the Almighty is represented as in any way coming under the obligation of an oath (; ). Instead of the head, the phylactery was sometimes touched by the Jews on taking an oath.
The levity of the Jewish nation in regard to oaths, though reproved by some of their doctors, was notorious; and their conduct in this respect was severely censured by Christ himself in language which seems to forbid the use of oaths altogether (; ).
Kitto, John, ed. Entry for 'Oath'. "Kitto's Popular Cyclopedia of Biblial Literature". https://www.studylight.org/encyclopedias/eng/kbe/o/oath.html.