On the necessary points, unity. On the questionable points, liberty. In everything, love.

[In necessariis unites, in non necessariis libertas, in omnibus caritas.]

Rupertus Meldenius (1582-1651) German writer [pseud. of Peter Meiderlin]

Paraenesis Votiva pro Pace Ecclesiae ad Theologos Augustanae Confessionis [Votive Counsel for the Peace of the Church, to the Theologians of the Augustan Confession] (1626)

Also translated as "essentials" and "non-essentials."

Paraphrase of final lines of the work:

Verbo dicam: Si nos servaremus IN necesariis Unitatem, IN non-necessariis Libertatem, IN UTRISQUE Charitatem, optimo certe loco essent res nostrae.

[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.]

Commonly attributed to St Augustine, but also to John Wesley, Richard Baxter, and several others.