Commentaries:
God uses the example of a man who divorces his wife, and then she marries another man. At some point, she decides she does not like her second husband and returns to her first husband. The law of God in the Pentateuch forbids such a thing. However, God is saying that His approach is this: His mercy is so great that, even under such a circumstance, He will forgive His wife and accept her back. The subject is marriage and divorce, in this case between God and Israel.
John W. Ritenbaugh
Love and Works
Jeremiah wrote this over 400 years after Israel's rejection of God as King and about 840 years after making the covenant at Mount Sinai. Even though by the time of this writing God had divorced the Great Harlot Israel, He still continued to have a fractious relationship with her in order to continue the outworking of His purpose and to fulfill His promises to Abraham, including all the end-time prophecies. In other words, He was not yet finished with Israel.
John W. Ritenbaugh
The Beast and Babylon (Part Eight): God, Israel, and the Bible
Other Forerunner Commentary entries containing Jeremiah 3:1:
Amos 5:1-3