Hebrews 3:19

They did not believe God could bring them into the promised land in the face of powerful opposition and so they would not attempt its conquest when commanded to go forward. They were rendered weak by their unbelief. This is pointed out in the concluding words καὶ βλέπομεν … where the emphasis is on οὐκ ἠδυνήθησαν, they were not able to enter in, the reason being given in the words διʼ ἀπιστίαν. The application to the Hebrew Christians was sufficiently obvious. They were in danger of shrinking from further conflict and so losing all they had won. They had begun well but were now being weakened and prevented from completing their victory; and this weakness was the result of their not trusting God and their leader.