Have you ever heard the saying "unity in necessary things; liberty in doubtful things; charity in all things" or its Latin equivalent "In necessariis unitas, in dubiis libertas, in omnibus caritas"? It has often been attributed to Augustine or to John Wesley or to Richard Baxter or to John Comenius. It is a common saying, employed directly or in abbreviated form for instance by the 19th-century Stone-Campbell (Restoration) Movement, by Methodists, and in Pope John XXIII's 1959 encyclical, Ad Petri cathedram, and it is the motto of organizations ranging from the Evangelical Presbyterian Church to the Moravian Church to the National Grange.
So where does this expression really come from? The source is an irenical 17th-century German Lutheran by the name of Peter Meiderlin (Latinized: Petrus Meuderlinus), who also went by the pseudonym Rupertus Meldenius. During the strife of the Thirty-Years War, he wrote a Latin tract called Paraenesis votiva pro pace ecclesiae ad theologos Augustanae confessionis auctore Ruperto Meldenio Theologo (1626) encouraging theologians in the midst of their controversies to not forget love, which included the closing words: "In a word, were we to observe unity in essentials, liberty in incidentals, and in all things charity, our affairs would be certainly in a most happy situation." This phrase has been repeated many times and attributed to many sources. But the origins of this quote have been documented by the following 19th-century scholars: Jan van der Hoeven, Gottfried Christian Friedrich Lücke, Philip Schaff (History of the Christian Church, Vol. 7, pp. 885). Knowing the source provides the historical context necessary to properly understand this common saying.