When God made promise to Abraham - The promise referred to is that made to Abraham when he had offered his son Isaac on the altar, Genesis 22:16-18; : "By myself have I sworn, saith the Lord; for because thou hast done this thing, and hast not withheld thy son, thy only son; that in blessing I will bless thee, and in multiplying I will multiply thy seed as the stars of the heaven, and as the sand which is upon the seashore; and thy seed shall possess the gate of his enemies; and in thy seed shall all the nations of the earth be blessed." Of this promise the apostle only quotes a part, as is generally the case, because he knew that his readers were well acquainted with the Scriptures of the Old Testament, and particularly with the law.
He sware by himself - He pledged his eternal power and Godhead for the fulfillment of the promise; there was no being superior to himself to whom he could make appeal, or by whom he could be bound, therefore he appeals to and pledges his immutable truth and Godhead.
For when God made promise to Abraham - That he would bless him, and multiply his seed as the stars of heaven; Genesis 22:16-17. The object of introducing this example here is, to encourage those to whom the apostle was writing to persevere in the Christian life, This he does by showing that God had given the highest possible assurance of his purpose to bless his people, by an oath. Reference is made to Abraham in this argument, probably, for two reasons:
(1)To show the nature of the evidence which Christians have that they will be saved, or the ground of encouragement - being the same as that made to Abraham, and depending, as in his case, on the promise of God; and,
(2)because the “example” of Abraham was just in point. He had persevered. He had relied firmly and solely on the promise of God. He did this when appearances were much against the fulfillment of the promise, and he thus showed the advantage of perseverance and fidelity in the cause of God.
Because he could swear by no greater - There is no being greater than God. In taking an oath among people it is always implied that the appeal is to one of superior power, who is able to punish for its infraction. But this could not occur in the case of God himself. There was no greater being than himself, and the oath, therefore, was by his own existence.
He sware by himself - Genesis 22:16. “By myself have I sworn;” compare Isaiah 45:23. In an oath of this kind God pledges his veracity; declares that the event shall be as certain as his existence; and secures it by all the perfections of his nature. The usual form of the oath is, “As I live, saith the Lord;” see Numbers 14:21, Numbers 14:28; Ezekiel 33:11.