Nicaea alone is sufficient; later councils are Arian offshoots
Athanasius and the bishops of Egypt and Libya are writing to the bishops of Africa, praising the recent synodal letters of Rome, Gaul, and Italy, and defending the unique authority of the Nicene council against those who prefer the council of Ariminum.
Of the ninety bishops of Egypt and Libya and of the blessed Athanasius, against the Arians, to the most honored bishops in Africa. What was written by our beloved fellow-minister Damasus, bishop of great Rome, and by the many bishops gathered with him, is sufficient; and no less sufficient are the acts of the other councils held in Gaul and in Italy concerning the sound faith, which Christ granted, the apostles preached, and the Fathers handed down—those who assembled at Nicaea from all the inhabited world known to us. For so great was the zeal at that time because of the Arian heresy, that those who had fallen into it might be pulled up, and those who had devised it might be exposed. At least by this means the whole world long ago agreed, and now, with many councils having been held, all those in Dalmatia, Dardania, Macedonia, Epirus, Greece, Crete, and the other islands, Sicily, Cyprus, Pamphylia, Lycia, Isauria, all Egypt and the Libyas, and very many in Arabia have recognized it. And they marveled at those who signed, because, even if any trace had remained among them—a bitterness growing up from the root of the Arians, I mean Auxentius, Ursacius, Valens, and those who think as they do—through these letters they were cut out and left behind.
What was confessed at Nicaea is sufficient and adequate, as we said before, both for the overthrow of every impious heresy and for the security and benefit of the church's teaching. But since we have heard that some, wanting to fight against it, attempt to name a certain council as having taken place at Ariminum, and contend that this one rather than that one should prevail, we considered it necessary to write and remind you, so that you may not tolerate such people. For this is nothing other than an offshoot of the Arian heresy again. For those who reject the council held against it—that is, the one held at Nicaea—what else do they want but for the teachings of Arius to prevail? What then are such people worthy of, but to be called Arians and to share the same penalty as those men? They neither feared God, who says, "Do not move the ancient boundary stone that your fathers set up" (Proverbs 22:28), and "Whoever curses his father or his mother shall be put to death" (Exodus 21:17); nor did they respect the Fathers, who commanded that those who think contrary to their confession be anathema.
For this reason the council at Nicaea became ecumenical, when three hundred eighteen bishops assembled concerning the faith because of the Arian impiety, so that splits might no longer occur piecemeal under the pretext of faith, but even if they are held, they might not prevail. For what does that council lack, that anyone should seek newer things? It is full of piety, beloved; it has filled the whole world. The Indians recognized it, and all those among the other barbarians who are Christians. Therefore the labor of those who have often attempted against it is vain. For such people have already made ten or more councils, changing their position at each one, taking away some things from the earlier ones, and altering and adding other things in the later ones.
Greek
ΤΩΝ ΚΑΤ' ΑΙΓΥΠΤΟΝ ΚΑΙ ΛΙΒΥΗΝ ΕΠΙΣΚΟΠΩΝ ΕΝΕΝΗΚΟΝΤΑ ΚΑΙ ΤΟΥ ΜΑΚΑΡΙΟΥ ΑΘΑΝΑΣΙΟΥ ΚΑΤΑ ΑΡΙΑΝΩΝ ΠΡΟΣ ΤΟΥΣ ΕΝ ΤΗ ΑΦΡΙΚΗ ΤΙΜΙΩΤΑΤΟΥΣ ΕΠΙΣΚΟΠΟΥΣ Ἱκανὰ μὲν τὰ γραφέντα παρά τε τοῦ ἀγαπητοῦ καὶ συλλειτουργοῦ ἡμῶν Δαμάσου, τοῦ ἐπισκόπου τῆς μεγάλης Ῥώμης, καὶ τῶν σὺν αὐτῷ τοσούτων συνελθόντων ἐπισκόπων· οὐδὲν δὲ ἧττον καὶ τὰ τῶν ἄλλων γενομένων συνόδων ἔν τε τῇ Γαλλίᾳ καὶ τῇ Ἰταλίᾳ περὶ τῆς ὑγιαινούσης πίστεως, ἣν ὁ μὲν Χριστὸς ἐχαρίσατο, οἱ δὲ ἀπόστολοι ἐκήρυξαν, καὶ οἱ Πατέρες παραδεδώκασιν οἱ ἐν τῇ Νικαίᾳ συν ελθόντες ἀπὸ πάσης τῆς καθ' ἡμᾶς οἰκουμένης. Τοσαύτη γὰρ γέγονε τότε σπουδὴ διὰ τὴν Ἀρειανὴν αἵρεσιν, ἵνα οἱ μὲν πεπτωκότες εἰς αὐτὴν ἀνασπασθῶσιν, οἱ δὲ ἐφευρόντες ἔκδηλοι γένωνται. Ταύτῃ γοῦν καὶ πάλαι πᾶσα ἡ οἰκουμένη συμπεφώνηκε, καὶ νῦν δὲ πολλῶν συνόδων γενομένων, ὑπομνησθέντες πάντες οἵ τε κατὰ τὴν Δαλματίαν, καὶ Δαρδανίαν, καὶ Μακεδονίαν, Ἠπείρους τε καὶ τὴν Ἑλλάδα, καὶ Κρήτην, καὶ τὰς ἄλλας νήσους, Σικελίαν τε καὶ Κύπρον, καὶ Παμφυλίαν, Λυκίαν τε καὶ Ἰσαυρίαν, καὶ πᾶσάν τε τὴν Αἴγυπτον καὶ τὰς Λιβύας, καὶ πλεῖστοι τῶν ἐν τῇ Ἀραβίᾳ ταύτην ἐπέγνωσαν· καὶ ἐθαύμασάν τε τοὺς ὑπογράψαντας, ὅτι, εἰ καί τι περιελέλειπτο παρ' αὐτοῖς ἐκτῆς ῥίζης τῶν Ἀρειανῶν ἄνω φύουσα πικρία, Αὐξέντιόν φαμεν καὶ Οὐρσάκιον, καὶ Οὐάλεντα καὶ τοὺς τὰ αὐτὰ φρονοῦντας αὐτοῖς, διὰ τῶν γραμμάτων τούτων ἐξεκόπησαν καὶ ἀπελείφθησαν. Ἱκανὰ μὲν οὖν τὰ ἐν τῇ Νικαίᾳ ὁμολογηθέντα, καὶ αὐτάρκη, καθὰ προείπομεν, πρός τε ἀνατροπὴν πάσης ἀσεβοῦς αἱρέσεως, καὶ πρὸς ἀσφάλειαν καὶ ὠφέλειαν τῆς ἐκκλησιαστικῆς διδασκαλίας. Ἐπειδὴ δὲ ἠκούσαμεν, ὅτι τινὲς, βουλόμενοι μάχεσθαι πρὸς αὐτὴν, ὀνομάζειν ἐπιχειροῦσι σύνοδόν τινα ὡς ἐν Ἀριμήνῳ γενομένην, καὶ φιλονεικοῦσι ταύτην μᾶλλον ἢ ἐκείνην κρατεῖν· ἀναγκαῖον ἡγησάμεθα γράψαι καὶ ὑπομνῆσαι ὑμᾶς, ἵνα μὴ ἀνέχησθε τῶν τοιούτων· τοῦτο γὰρ οὐδὲν ἕτερόν ἐστιν ἢ πάλιν τῆς Ἀρειανῆς αἱρέσεως παραφυάς. Οἱ γὰρ τὴν κατ' αὐ τῆς σύνοδον γενομένην παραιτούμενοι (ἔστι δὲ ἡ ἐν Νικαίᾳ γενομένη), τί ἕτερον βούλονται, ἢ τὰ Ἀρείου κρατεῖν; Τίνος οὖν ἄξιοι οἱ τοιοῦτοι ἢ Ἀρειανοὶ μὲν καλεῖσθαι, τῆς αὐτῆς δὲ ἐπιτιμίας ἐκείνοις μετασχεῖν; οἳ μήτε τὸν Θεὸν ἐφοβήθησαν λέγοντα· Μὴ μέταιρε ὅρια αἰώνια, ἃ ἔθεντο οἱ πατέρες σου· καὶ, Ὁ κακολογῶν πατέρα ἢ μητέρα θανάτῳ τελευτάτω· οἳ μήτε τοὺς Πατέρας ᾐδέσθησαν, παραγγείλαντας ἀνάθεμα εἶναι τοὺς τὰ ἐναντία φρονοῦντας τῆς ὁμολογίας αὐτῶν. Διὰ τοῦτο γὰρ καὶ οἰκουμενικὴ γέγονεν ἡ ἐν Νι καίᾳ σύνοδος, τριακοσίων δέκα καὶ ὀκτὼ συνελθόντων ἐπισκόπων περὶ τῆς πίστεως διὰ τὴν Ἀρειανὴν ἀσέβειαν· ἵνα μηκέτι κατὰ μέρος προφάσει πίστεως γίνωνται· ἀλλὰ, κἂν γένωνται, μὴ κρατῶσι. Τί γὰρ ἐκείνῃ λείπει, ἵνα καινότερα ζητήσῃ τις; Πλήρης ἐστὶν εὐσεβείας, ἀγαπητοί· αὕτη πᾶσαν τὴν οἰκουμένην πεπλήρωκε. Ταύτην ἔγνωσαν καὶ Ἰνδοὶ, καὶ ὅσοι παρὰ τοῖς ἄλλοις βαρβάροις εἰσὶ Χριστιανοί. Οὐκοῦν μάταιος ὁ κάματος τοῖς κατ' αὐτῆς πολλά κις ἐπιχειρήσασιν. Ἤδη γὰρ οἱ τοιοῦτοι δέκα καὶ πλέον που συνόδους πεποιήκασι, καθ' ἑκάστην μεταβαλλόμενοι, καὶ τὰ μὲν ἀπὸ τῶν πρώτων ἀφαιροῦντες, τὰ δὲ ταῖς μετὰ ταῦτα ἐναλλάσσοντες καὶ προστιθέντες.