In necessariis unitas, in dubiis libertas, in omnibus caritas is a Latin phrase commonly translated as "unity in necessary things; liberty in doubtful things; charity in all things", or, more literally, "in necessary things unity; in uncertain things freedom; in everything compassion".
It is often misattributed to St. Augustine of Hippo, but seems to have been first used in the 17th century by the Archbishop of Split (Spalato) Marco Antonio de Dominis (1560-1624), in book 4, chapter 8 (p. 676 of the first volume) of his De republica ecclesiastica libri X (London, 1617), where it appears in context as follows: Quod si in ipsa radice, hoc est sede, vel potius solio Romani pontificis haec abominationis lues purgaretur et ex communi ecclesiae consilio consensuque auferretur hic metus, depressa scilicet hac petra scandali ac ad normae canonicae iustitiam complanata, haberemus ecclesiae atrium aequabile levigatum ac pulcherrimis sanctuarii gemmis splendidissimum. Omnesque mutuam amplecteremur unitatem in necessariis, in non necessariis libertatem, in omnibus caritatem. Ita sentio, ita opto, ita plane spero, in eo qui est spes nostra et non confundemur. Ita sentio, ita opto, ita plane spero, in eo qui est spes nostrae et non confundemur. This according to a 1999 article by H. J. M. Nellen ("De zinspreuk 'In necessariis unitas, in non necessariis libertas, in utrisque caritas,'" Nederlands archief voor kerkgeschidenis 79, no. 1 (1999): 99-106 (with abstract in English)), an article that overturned a century or more of scholarly consensus.
Prior to the appearance of the article by Nellen, the sole serious candidate had been the German Lutheran theologian Peter Meiderlin (also known as Rupertus Meldenius), who, in his Paraenesis votiva pro pace ecclesiae ad theologos Augustanae of 1626 had said, "Verbo dicam: Si nos servaremus in necesariis Unitatem, in non-necessariis Libertatem, in utrisque Charitatem, optimo certe loco essent res nostrae.", meaning "In a word, let me say: if we might keep in necessary things Unity, in unnecessary things Freedom, and in both Charity, our affairs would certainly be in the best condition".
According to Joseph Lecler, the substitution of dubiis for non necessariis (note also that omnibus occurs here, rather than, as in Meiderlin, utrisque) was made in largely Catholic circles, and had the effect of extending "the rule of Meldenius . . . to much more than just the necessaria [(for salvation)] and the non necessaria [(for salvation)]", much more than just the "fundamental articles": "the tripartite maxim. . . . [thus] lost its original Protestant nuance, in order to extend liberty to the entire domain of questions debated, doubtful, and undefined [(non définies par l'Église)]" (Joseph Lecler, "À propos d'une maxime citée par le Pape Jean XXIII: In necessariis unitas, in dubiis libertas, in omnibus caritas," Recherches de science religieuse 49 (1961): 549-560). But Lecler was reproducing the old consensus: that the maxim originated in proto-Pietistic rather than Catholic circles, i.e. the circle about Johann Arndt.
Richard Baxter was apparently the theologian responsible for its dissemination throughout the English-speaking world.
The maxim is widely quoted in defence of theological and religious freedom, even though it raises the essential question of which things are necessary and which are doubtful or unnecessary.
It is also the motto of the Moravian Church and the Evangelical Presbyterian Church (United States), as well as the Cartellverband der katholischen deutschen Studentenverbindungen, ÖCV and CV, and the Unitas Verband der Wissenschaftlichen Katholischen Studentenvereine, UV and UVÖ the associations of Catholic student fraternities of Austria and Germany. The phrase in its current form is found in Pope John XXIII's encyclical Ad Petri Cathedram [1].