The Inability of God to Lie
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" magnifies His greatness and brings 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." The Lord wants to assure us that He will not declare one thing, and then later change His mind and do something else.
In order to provide us with solid assurance, God coupled His promise with an oath ("confirmed it by an oath"). People make oaths, attempting to convince others of their reliability. They swear by something greater than themselves. "For men indeed swear by the greater" (Hebrews 6:16). However, "when God made a promise to Abraham, because He could swear by no one greater, He swore by Himself, saying, 'Surely blessing I will bless you, and multiplying I will multiply you'" (Hebrews 6:13-14). This marks an amazing condescension on the part of our Lord toward us. We might say that He somewhat lowers Himself down to our level (which He would fully do in the incarnation, becoming a man). He uses a common human custom to grant us an assured understanding of His reliable commitment to us.
In this unusual communication from God, we receive assurance that is likened unto a "double certainty." The "God, who cannot lie," makes a promise and an oath, "that by two immutable things…we might have strong consolation."