For when God made promise to Abraham, because he could swear by no greater, he sware by himself, 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.