Romans 4v17

As it is written: "I have made you a father of many nations." He is our father in the sight of God, in whom he believed--the God who gives life to the dead and calls things that are not as though they were.

God calls things that are not as though they are. This is part of what faith is calling things as though they are. Abraham was not a father but God called him a "father of nations" and it became so. I think that part of us having faith in God is that we have to start by calling somethings as though they are - even though they don't seem possible at the moment.