The Israelites were in possession of the land; not one word of God had failed; He had with Himself given them all things; but they did not appreciate the wondrous gift, and so received the grace of God in vain.
They were at least nominally faithful to God during the life of Joshua, but after his death “the children of Israel did evil in the sight of the Lord, and served Baalim; and they forsook the Lord God of their fathers, which brought them out of the land of Egypt, and followed other gods, of the gods of the people that were round about them, and bowed themselves unto them, and provoked the Lord to anger. And they forsook the Lord, and served Baal and Ashtaroth. And the anger of the Lord was hot against Israel, and He delivered them into the hands of spoilers that spoiled them, and He sold them into the hands of their enemies round about, so that they could not any longer stand before their enemies. Whithersoever they went out, the hand of the Lord was against them for evil, as the Lord had said, and as the Lord had sworn unto them; and they were greatly distressed.” God told them that because of their disobedience He would not drive the nations out from before them, but that their enemies should remain and be as thorns in their sides. Judges 2.1-15
Thus we see that although God gave them rest, they did not enter into it. It was therefore as true of them as of those who fell in the wilderness that “they could not enter in because of unbelief.”
“Let us therefore fear, lest, a promise being left us of entering into His rest, any of you should seem to come short of it. For unto us was the Gospel preached, as well as unto them; but the word preached did not profit them, not being mixed with faith in them that heard it.” Hebrews 4.1, 2. We are in the world in precisely the same situation that ancient Israel was, with the same promises, the same prospects, the same enemies, the same dangers.