Charles Ellicott Commentary Jeremiah 3:18

"In those days the house of Judah shall walk with the house of Israel, and they shall come together out of the land of the north to the land that I gave for an inheritance unto your fathers." — Jeremiah 3:18 (ASV)

In those days ... —As with Isaiah (Isaiah 11:13), so with Jeremiah, the hope, however distant, of national reformation was connected with that of a restoration of national unity. The healing of the long-standing breach between Israel and Judah, almost contemporary with the beginning of Israel as a people, was to be the glory of the Messiah’s kingdom.

Out of the land of the north. —The thoughts of the prophet turn mainly to the land of the exile of the ten tribes; but his words imply that he also foresees a similar exile in the north for Judah. In that distant land, the house of Judah will walk to (rather than with) the house of Israel, seeking its alliance, asking for reconciliation, and both would once again dwell in the land of their inheritance.