Joshua 23:8

“ But cleave to YHWH your God, as you have done to this day.”

Up to this time they had been faithful. They had driven out the Canaanites. They had spurned their gods. They had been faithful to the tribal covenant, and gathered at the central sanctuary as required by the Law. They had not settled down and fraternised with those around them. For their leaders (the elders that outlived Joshua - Judges 2:7) had kept them faithful. This they must continue to do.