In the same way God, desiring even more to show to the heirs of the promise the unchangeableness of His purpose, interposed with an oath,
so that by two unchangeable things in which it is impossible for God to lie, we who have taken refuge would have strong encouragement to take hold of the hope set before us. (Hebrews 6:17-18)
One benefit of living by the promises of God is related to something that God cannot do. He is unable to lie. “It is impossible for God to lie.” This “inability” actually magnifies His greatness, while bringing to us great assurance.
This “inability” of God is linked here to His promises. We who live by faith are “heirs of promise.” We inherit the blessings of God by trusting Him to fulfill all that He has promised to do. These promises offer everlasting life and are anchored in eternity past: “In hope of eternal life which God . . . promised before time began.” Now, here in time and space, God wants to deeply impress us with the unchangeable character of His will: “Thus God, determining to show more abundantly to the heirs of promise the immutability of His counsel.” He wants us to be fully assured that He will not declare one thing, and then later change His mind and do something else.
Excerpt from “Day By Day Grace – Bob Hoekstra