"Faith is being sure of what we hope for and certain of what we do not see" (Heb 11:1 NIV). Assurance and certainty apply to "of what" a person has confidence in will come to pass. If it is "of things" (11:1 NASB), it is from, concerning or belonging to the existence of what is desired or expected. Hope must be based upon something potentially happening in the first place. When you are hopeful you are optimistic that your expectations are reasonable. But from a Biblical standpoint it is faith from God that makes hope authentic.
God told Abraham his descendants would be as the number of stars (Ge 15:5). But the time came when "Abraham and Sarah were old, advanced in age; Sarah was past childbearing" (18:11). In a natural way there was no substance for Abraham's hope. But God said "'Sarah your wife will have a son'" (:10). Therefore Abraham was "fully assured that what God had promised, He was able also to perform" (Ro 4:21). Abraham was aware of natural impossibilities because it was "against hope" (:18). No realistic person would hope in something impossible. But Abraham "in hope . . . believed . . . according to that which had been spoken" (:18). It was with scriptural hope based upon God's promise. Faith is the substance for hope because the assurance is inherent in the promise. Also, Abraham "with respect to the promise . . . did not waver in unbelief, but grew strong in faith, giving Glory to God" (:20). What God had spoken contained what convinced Abraham so that he could faithfully believe in it.