Because God wanted to make the unchanging nature of his purpose very clear to the heirs of what was promised, he confirmed it with an oath. God did this so that, by two unchangeable things in which it is impossible for God to lie, we who have fled to take hold of the hope offered to us may be greatly encouraged. (Hebrews 6:17-18, NIV)Context: Hebrews 6:16-20
While this verse says something about God being true, it also has an interesting phrase, "impossible for God". How do we understand this phrase in relation to Jesus saying, "...with God all things are possible" (Matthew 19:26, NIV; Mark 10:27, NIV)? We need to understand how the phrases are used. Since God can speak to us and has communicated with us through the Bible, we must admit that it is within God's ability to lie strictly from the standpoint of His power. However, it is impossible for God to lie because of His character. God is only limited by how He has chosen to limit Himself. God has the raw ability to be inconsistent, but He has chosen to be consistent. Thus, His eternal character is to be consistent and not to lie because He has eternally chosen to be that way, not because He is incapable of being anything else. Nothing is outside God's authority and control. However, many things are outside God's character, which no one imposes upon Him, but He imposes upon Himself.