"Things began to change among the Reformed. As the gulf between Lutherans and Reformed grew, an emphasis on personal regeneration began to grow among the Reformed. In my view, this was the result of the basic disagreement between Reformed and Lutheran on the sacraments. Calvin was closer to Luther than to Zwingli in his view of the Lord's Supper, but closer to Zwingli in what he believed ought to be the basis of ecclesiastical unity. That watershed was, in my view, highly significant, and helped to get us here." - Wilson, AC, p168.
That is a helpful summary to a much longer history lesson.