Hebrews 11:1 – “by faith even Sarah, who was past childbearing age, was enabled to bear children because she considered him faithful who had made the promise” (NIV)
It doesn’t say Sarah believed she was going to have a child and that’s why she was enabled to bear children. It says she considered God faithful who had made the promise.
Faith is more about the confident assurance in the one who promised and that his promise is Truth, rather than simply “believing” with the power of our own mind something to be true. In other words, it’s the object of our faith, not our mental faith alone.
How does this relate to being a co-heir with Christ? If I believe God’s promise that I am a co-heir with Christ and have the full legal authority of God’s son, then I will act in that confidence in what I do or declare.
Just like the policeman who goes into a vacant building to evict squatters. If he is timid and lacks confidence in the city’s power to back him up, he will be weak in evicting the squatters. However, if he is fully confident in the city’s power and willingness to back him up, he will act with great boldness and authority, evicting them immediately.
As a co-heir with Christ I have the backing of the full power and authority of God. If I consider God faithful in what he has promised in my adoption to sonship and all that it means, I need to walk in that confidence. I need to walk and live in the promise (truth), not to the promise (hope).