Rom 4:18

4:18 In hope he believed. Trusting in God’s power (v. 17), Abraham gained assurance that the promise would be fulfilled. Paul indicates that true faith is directed toward God and not humanity, toward the divine word and not toward the human situation.

against hope. In the natural course of events, believing that Sarah would bear his child (the first requirement for receiving what was promised) was utterly futile, for reasons given in v. 19.