In necessariis unitas, in non necessariis libertas, in utrisque caritas

"The fact that other customs and laws are kept by others, yet without violating the Faith or departing from common and universally-held decrees, will not lead the discerning observer into thinking either that those who keep them fall into the wrong, or that those who do not accept them violate the law."

"Where matters of faith are not denied and there is no case of falling away from the common and catholic teaching accepted by all, when some maintain different customs and uses, one should not condemn those who profess or accept them."

"Whenever that which is violated is not the faith, nor (there) is [(there)] a fall from the common and catholic decree, because other customs and laws are kept by others, he who knows how to judge rightly should not think that they who keep these fall into adikia or that they who do not accept them violate the law."

"When the faith remains inviolate, the common and catholic decisions are also safe."

Etc.

     Photius (St. Photios the Great), Patriarch of Constantinople, Epistle 290 to Pope Nicholas I, August/September 861.  See p. 131 ll. 241 ff. of vol. 3 of the Teubner edition ed. B. Laourdas & L. G. Westerink:

Οὕτως ἐν οἷς οὐκ ἔστι πίστις τὸ ἀθετούμενον,  οὐδὲ κοινοῦ τε καὶ καθολικοῦ ψηφίσματος ἔκπτωσις, ἄλλων παρ' ἄλλοις ἐθνῶν τε καὶ νομίμων φυλαττομένων, οὔτε τοὺς φύλακας ἀδικεῖν οὔτε τοὺς μὴ παραδεξαμένους παρανομεῖν ὀρθῶς ἄν τις κρίνειν εἰδὼς διορίσαιτο.

Obviously I have not yet translated that myself, but only reproduced the translations and paraphrases of it that I have found ready-to-hand.
     With thanks to Liz Leahy and her faculty for the diversion.

A cautionary note on 'image' and 'resemblance'

     "The text of Genesis to which Maximus refers[here, namely Gen 1:26,] juxtaposes 'image' and 'resemblance'.  The Gnostics had attributed to these two words a different sense:  the terrestrial, 'hylic' man was fashioned in the image of God; the 'psychic' man, in his resemblance.  It [was] perhaps this usage of the Gnostics that led also Irenaeus, their adversary, into a distinction of sense:  the image comprehends the natural gifts [of] reason [and] self-determination, and is inalienable [(inamisssible)]; the resemblance is given by possession of the Word and participation in the Spirit; lost in Adam, it is restored in Christ.  For Clement of Alexandria and Origen, image and resemblance are linked as power and act, imperfect state and consummation.  Athanasius, by contrast, abandons the Alexandrine tradition anterior [to him] and rejects every distinction between the two notions.  It is the same with Gregory of Nazianzus and with Basil.  In Gregory of Nyssa, the complexity of the question will allow him to say that the image is itself dynamic and that the resemblance is [on the other hand] ontological, [i.e.] not restricted to the operative order [of action alone].  The Antiochenes Theodore of Mopsuestia and Theodoret of Cyrus ignore the distinction [between] the two notions, as does Cyril of Alexandria.  But it becomes once again common after the homily [from the pen] of Pseudo-Gregory of Nyssa, Quid sit ad imaginem et similitudinem.
     "Many texts of Maximus [the Confessor] give expression to a dynamic straining of the natural image towards the free resemblance. . . .  'He who has allowed [(fait)] his mind to be illuminated [(étinceler)] by the flashes of lightning [(éclairs)] that divine contemplations diffuse, who has occupied his reason in the incessant offering of praise to the Creator, and who has purified his sensibility by means of irreproachable representations, this man adds to the natural good of the image the gnomic good of the resemblance.' . . .
     "But the word 'image' does not have only, in Maximus, the precise signification that we see in the preceding texts.  The absence of a break in continuity between the image and the resemblance permits him to pass from the one to the other, and to employ [often] the word 'image' [by itself] alone in the sense of resemblance[, and vice versa]. . . .
     "In the dithelite argumentation of the Dispute, [which was also] based on th[is very] text from Genesis, Maximus employs the two words 'image' and 'resemblance', but in a manner out of concord with the others in which he distinguishes the two notions[,] . . . [such that] the image, because it preserves its resemblance to the archetype, is naturally capable of self-determination [and] The resemblance does not signify here something alienable [(amissible)]; it adds nothing, it seems, to the image."

     Marcel Doucet, F.I.C., "La voluntaté humaine du Christ, spécialement en son agonie:  Maximus le Confesseur, interprète de l’écriture," Science et esprit 37, no. 2 (1985):  128-130 (123-159), following mostly W. J. Burghardt, The image of God in man according to Cyril of Alexandria (Woodstock, MD:  Woodstock College Press, 1957).