Jamieson, Fausset and Brown on Jeremiah 18:15

15. Because--rather, "And yet"; in defiance of the natural order of things.

forgotten me--(Jr 2:32). This implies a previous knowledge of God, whereas He was unknown to the Gentiles; the Jews' forgetting of God, therefore, arose from determined perversity.

they have caused ... to stumble--namely the false prophets and idolatrous priests have.

ancient paths--(Jr 6:16): the paths which their pious ancestors trod. Not antiquity indiscriminately, but the example of the fathers who trod the right way, is here commended.

them--the Jews.

not cast up--not duly prepared: referring to the raised center of the road. Calvin translates, "not trodden." They had no precedent of former saints to induce them to devise for themselves a new worship.