The narrator does not mean that the generation after Joshua did not know about the Lord or his great works. They could hardly be ignorant about God, or the facts of Israel’s history, like the exodus from Egypt, the crossing of the Jordan, the walls of Jericho falling, and the conquest of Canaan. At some level they would have been familiar with these works of the Lord, even if they could not speak from personal experience of the miracles God had performed, No, the issue is that they had no regard for the Lord and his great work, they cared nothing for him. In much the same way that the two godless sons of Eli, though priests at the Lord's sanctuary, did not know the Lord
(1 Samuel 2:12).1
God's saving acts "were no longer precious or central to them. They had not learned to revere and rejoice in what God had done. In other words, they had forgotten the 'gospel' that they were saved from slavery in Egypt and brought into the promised land by the gracious, mighty acts of God. Put simply, they forgot.”2 And this generation did not appreciate their freedom and rest in the Promised Land. To them the presence of the Canaanite survivors was not something abnormal that could only be endured temporarily.
3 This is a heart-breaking commentary.
10 And all that generation also were gathered to their fathers. And there arose another generation after them who did not know the LORD or the work that he had done for Israel.