“Then will they know that I am Yahweh, when I have made the land a desolation and an astonishment, because of all their abominations which they have committed.”
The people who remained continued in the abominations described above. So instead of returning to Yahweh and listening to Jeremiah, assassination and warfare continued among those remaining, and those who joined them. There was a short civil war and then those who remained left for Egypt taking Jeremiah with them (Jeremiah 41-43). Thus they learned Who and What Yahweh was, the moral Overlord, the One Who would not bless and protect them while they sinned against Him and His covenant.