Unity in things Necessary, Liberty in things Unnecessary, and Charity in all. Rupertus Meldenius (Peter Meiderlin) in Paraenesis votiva pro Pace Ecclesiae ad Theologos Augustanae Confessionis, Auctore Ruperto Meldenio Theologo (c. 1627). Some forms of this statement have been attributed to Richard Baxter, but though he used the motto, he himself attributed it to Meldenius.
The first known occurence of such an expression is as "Omnesque mutuam amplecteremur unitatem in necessariis, in non necessariis libertatem, in omnibus caritatem" in De Republica Ecclesiastica by Marco Antonio de Dominis, Pars I. London (1617), lib. 4 cap. 8 p. 676 (penultimate sentence) books.google, cf. liberlocorumcommunium