"When God made a promise to Abraham, because He could swear by no one greater, He swore by Himself." By these words the Apostle points to what God said to Abraham after the sacrifice of Isaac: "By Myself I have sworn. Because you have done this thing, and have not withheld your son, your only son, for My sake, blessing I will bless you, and multiplying I will multiply your seed" (Gen 22:16-17).
"Men indeed swear by the greater, and an oath for confirmation is for them an end of every dispute." By the greater, κατὰ τοῦ μείζονος, the Lord is meant here. This is why an oath has confirmatory force. This is why that force extends to every dispute among them, so that an oath removes every possibility of contradiction.
"Thus God, desiring to show the immutability of His counsel, interposed with an oath." To make these last words clearer, one must know that the Slavonic says a verb meaning "acted as mediator," while the Greek has ἐμεσίτευσεν, from "mediator," μεσίτης. Both mean that He became mediator through an oath.
A mediator is a person who strives to reconcile two hostile or disputing sides, or who helps negotiations between people divided by distance and mutual difference. When God mediated by an oath, it means that He placed Himself between man and Himself.
This idea is very proper to Scripture. According to its teaching, God is advocate, surety, and mediator between man and Himself. Thus Job cried out, turning to God: "Lay down a pledge for me with Yourself; who is there who will pledge himself for me?" (Job 17:3).
If God placed Himself as mediator in that unique oath when He swore by Himself, then clearly He appears as the same mediator and surety when He does not swear by Himself, but men swear by Him as the greater.
From this the true understanding of an oath becomes clear. It consists in this: when men assure one another, they call God as witness, mediator, and surety. They do this with the thought that He will help fulfill the promise and punish intentional violation or false assurance.
What impious or irreverent thing can be found in such an oath? God Himself swore, though He had no one by whom to swear except Himself. His promises, being incapable of falsehood, needed no confirmation by oath. Distrust of them would have been a crime on man's part.
Therefore an oath is all the more fitting among men. Human words can be true insofar as they are accompanied by thought of the Most High, the all-knowing God, who is the highest Truth and hates all who speak falsehood (Ps 5:6).
As soon as the remembrance of God leaves a man, his mouth and heart are filled with falsehood. Whoever in an oath calls God as mediator and surety of his promises or assurances thereby confesses a reverent remembrance of Him as the Knower of hearts.
He confesses Him as the One who knows that his words are sincere and that his promises contain no deceit. It is therefore understandable why, as the Apostle says, "an oath," used for confirmation, "ends every dispute" among men.