God and Israel's relationship began like a marriage. God vowed to be faithful, and Israel promised to be lovingly his.
But Israel repeatedly broke her marriage vows. She sacrificed to false gods; she even killed her own children in their rituals. Jeremiah called Israel a whore for this idolatry. When Israel worshiped other gods, she was really committing adultery and Jeremiah begged for her to return and repent.
But Israel wouldn't listen, she chose to break her vows and her covenant with God. But God promised that he would be a faithful husband to his adulterous bride. In Jesus, he would begin a new covenant that would transform his people from adulterous whores and make them virgin brides once again.