Yahweh’s answer to their prayer in Jeremiah 3:21 is immediately followed by their acceptance of the offer of divine mercy.
For - Rather, because ... This profession of faith gives the reason why they return to Yahweh. The whole description is most graphically conceived. The people weeping upon the hills: God’s gracious voice bidding them return: the glad cry of the penitents exclaiming that they come: the profession of faith won from them by the divine love; these form altogether a most touching picture of a national repentance.