Jamieson, Fausset and Brown on Jeremiah 3:6

6. Jr 3:6-6:30, is a new discourse, delivered in Josiah's reign. It consists of two parts, the former extending to Jr 4:3, in which he warns Judah from the example of Israel's doom, and yet promises Israel final restoration; the latter a threat of Babylonian invasion; as Nabopolassar founded the Babylonian empire, 625 B.C., the seventeenth of Josiah, this prophecy is perhaps not earlier than that date (Jr 4:5, &c.; Jr 5:14, &c.; Jr 6:1, &c.; Jr 22:1-30); and probably not later than the second thorough reformation in the eighteenth year of the same reign.

backsliding--literally, "apostasy"; not merely apostate, but apostasy itself, the essence of it (Jr 3:14, 22).