Everybody has a natural, human faith. When you walk into a church you don't ask the ushers if the seat will hold you up. You would be surprised if it fell through when you sat down. When you were born again you received God's measure of faith in you. So you now have the ability to believe for supernatural things.
Spiritual things are more real than natural.
You will never sit on God's seat of faith and fall through. Yet we are surprised when our prayers work. It is the same with hope. Verse 18 is saying that against all natural hope Abraham found a supernatural hope. In the natural there was no way for him and Sarah to have a child, but God gave him a new hope.