Three things are said to be unchangeable: God's counsel, His word, and His oath. Heb 6:17-18. To these certainty may be attached. In Mal 3:6 we read, "I am Jehovah, I change not," and the Lord is "the same yesterday, and to-day, and for ever." Heb 13:8.
Since God wanted to show conclusively to those who would inherit the promise [i.e., Abraham and all of his spiritual descendants], how unchangeable His purpose was, He confirmed it with an oath. He did this so that by two unchangeable things [i.e., His promise and His oath], (and it is impossible for God to lie), we, who have fled [to God] for refuge, could be greatly encouraged to take hold of the hope set before us [i.e., the hope of never ending life].
Jesus Christ is the same yesterday and today and forever.