Gr. mesiteuō, “to mediate,”“to act as surety,”“to guarantee.” The confirmation of God’s word by an oath is recorded in Gen. 22:16-18.
Immutability of his counsel.
Or, “unchangeableness of His purpose.”
Unto the heirs.
Not only Abraham but all the heirs including the spiritual descendants of Abraham (Gal. 3:29).
Willing … to shew.
It was not necessary for God to swear. His word is as good as His oath. It was a wonderful thing that He would so put Himself on a level with man that He would consent to swear to the truth of the promise.