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

Augustine of Hippo (354-430) Christian church father, philosopher, saint [b. Aurelius Augustinus]

(Misattributed)

A commentary on theological / doctrinal dispute, frequently attributed to Augustine, but not found in his works.

The first known occurrence of such an expression is in Marco Antonio de Dominis, De Republica Ecclesiastica, Book 4, ch. 8, penultimate sentence (1617):

Omnesque mutuam amplecteremur unitatem in necessariis, in non necessariis libertatem, in omnibus caritatem.

[And let us all embrace one another, unity in what is necessary, liberty in what is not necessary, charity in all things.]

The phrase was also adapted by Richard Baxter (1615-1691) as his motto. See also Rupertus Meldenius (1626).

More discussion about this quotation here: Liber locorum communium: In necessariis unitas, in non necessariis libertas, in utrisque caritas (Marco Antonio De Dominis, 1617), cf. In necessariis unitas, in dubiis libertas, in omnibus caritas (and other variants). English: "In essentials unity ..."