Origin of the phrase, ‘In necessariis unitas’

Like me, you've probably heard some variation of the phrase, "In essentials, unity; in non-essentials, liberty; in all things, charity." It's a translation of the Latin phrase, "In necessariis unitas, in dubitas libertas, in omnibus caritas." Nineteenth-century church historian Philip Schaff calls it "the watchword of Christian peacemakers."(1) The phrase is often attributed to the theologian …