My father once was asked to lecture on the question of whether the God of the Old Testament and of the New Testament is the same. He started by quoting dozens of Scripture passages proving that the God of the Old Testament was a God of love and mercy who would set aside His vengeance. Then he turned to the book of Revelation showing that the God of the New Testament is a God of vengeance who never will love His enemies. Then he asked his listeners how we could ever harmonize the loving God of the Old with the judging God of the New Testament. The listeners at first were confused but soon understood the lesson.
Schirrmacher, T. (1992). “Love is the Fulfilment of Law: With the Law Dies Love” in Calvinism Today.