Deuteronomy 1:34

Ver. 34. The Lord—was, and sware— Moses makes God speak in the manner of the kings of the earth; and that, to accommodate himself to the feeble reach of our understanding. That God cannot be in a passion, is certain; when the Scripture represents him in this light, it is the better to make us comprehend how much he detests evil. In the same manner, if he is introduced swearing, it is to give the greater force and strength to his asseverations, agreeable to those forms which are established among men. It is well known, that the Pagans supposed that their gods might swear: even their supreme god Jupiter, as well as the rest; which shews, that the general idea which mankind have affixed to the term swearing means no more than giving the strongest and most awful assurances possible, and does not necessarily imply the invoking a superior. See Dr. Waterland's Script. Vind. part 2: p. 47 and Genesis 6:6.