Jeremiah 3:6-25 ; Jeremiah 4:1-4 is Jeremiah's Fifth prophecy. (see Book comments for Jeremiah).
in the days of Josiah. This must be noted to understand the context.
Israel. Here refers to the Northern Kingdom. In Jeremiah it usually refers to the whole nation.
mountain... tree. Compare Jeremiah 2:20, and Hosea 4:13.
played the harlot. The whole of this refers to idolatry, chiefly because of the uncleanness connected with the phallic worship of the Canaanitish nations.