Sadly little known in our day, David Pareus was a major figure. He taught at the University of Heidelberg in the early 17th century and labored particularly towards Protestant ecumenism. He was able to reduce the disagreement in doctrine between the Reformed and Lutheran to one point:
Building directly on the precedent established at the colloquy of Marburg in 1529, Pareus reduced the disputed points between Evangelicals and Reformed to a single article: the Lord’s Supper…
Pareus, for example, distinguished between “articuli catholic,” which form the foundation of faith and salvation and must therefore be taught to all Christians, and “articuli theologici,” which pertain to theological knowledge proper to the profession of theologians but are not part of saving faith. The few questions separating the Evangelical churches belong to the category of inessential, non-fundamental, “theological” articles, which are not legitimate grounds for dividing the churches.
( Howard Hotson, “Irenicism in the Confessional Age,” Conciliation and Confession. pg. 236)