Speaking Attic Greek makes you Athenian (with the right customs)

Thucydides, History 7.57.2:
Ἀθηναῖοι μὲν αὐτοὶ Ἴωνες ἐπὶ Δωριᾶς Συρακοσίους ἑκόντες ἦλθον, καὶ αὐτοῖς τῇ αὐτῇ φωνῇ καὶ νομίμοις ἔτι χρώμενοι Λήμνιοι καὶ Ἴμβριοι καὶ Αἰγινῆται, οἳ τότε Αἴγινανεἶχον, καὶ ἔτι Ἑστιαιῆς οἱ ἐν Εὐβοίᾳ Ἑστίαιαν οἰκοῦντες ἄποικοι ὄντες ξυνεστράτευσαν.

The Athenians themselves, as Ionians, came over against Dorian Syracuse of their own free will. They were joined in the expedition by the Lemnians, the Imbrians, the Aeginetans occupying Aegina at the time, and also the Hestiaeans settled at Hestiaea in Euboea: all these were Athenian colonists speaking the same dialect as the Athenians and retaining the same institutions.

Thucydides, History 7.63.3-4:
τοῖς δὲ ναύταις παραινῶ καὶ ἐν τῷ αὐτῷ τῷδε καὶ δέομαι μὴ ἐκπεπλῆχθαί τι ταῖς ξυμφοραῖς ἄγαν, τήν τε παρασκευὴν ἀπὸ τῶν καταστρωμάτων βελτίω νῦν ἔχοντας καὶ τὰς ναῦς πλείους, ἐκείνην τε τὴν ἡδονὴν ἐνθυμεῖσθαι ὡς ἀξία ἐστὶ διασώσασθαι, οἳ τέως Ἀθηναῖοι νομιζόμενοι καὶ μὴ ὄντες ἡμῶν τῆς τε φωνῆς τῇ ἐπιστήμῃ καὶ τῶν τρόπων τῇ μιμήσει ἐθαυμάζεσθε κατὰ τὴν Ἑλλάδα, καὶ τῆς ἀρχῆς τῆς ἡμετέρας οὐκ ἔλασσον κατὰ τὸ ὠφελεῖσθαι ἔς τε τὸ φοβερὸν τοῖς ὑπηκόοις καὶ τὸ  μὴ ἀδικεῖσθαι πολὺ πλέον μετείχετε.  
[4] ὥστε κοινωνοὶ μόνοι ἐλευθέρως ἡμῖν τῆς ἀρχῆς ὄντες δικαίως [ἂν] αὐτὴν νῦν μὴ καταπροδίδοτε, καταφρονήσαντες δὲ Κορινθίων τε, οὓς πολλάκις νενικήκατε, καὶ Σικελιωτῶν, ὧν οὐδ’ ἀντιστῆναι οὐδεὶς ἕως ἤκμαζε τὸ ναυτικὸν ἡμῖν ἠξίωσεν, ἀμύνασθε αὐτούς, καὶ δείξατε ὅτι καὶ μετ’ ἀσθενείας καὶ ξυμφορῶν ἡ ὑμετέρα ἐπιστήμη κρείσσων ἐστὶν ἑτέρας εὐτυχούσης ῥώμης.

As for the sailors, I urge you, and not only urge but beg you not to take your previous reverses too much to heart: you have better resources on deck now, and a greater number of ships. Some of you are not citizens, but so far have been regarded as honourary Athenians and admired for that throughout the Greek world, because you speak our language and have assimilated our culture. Remember what a joy that privilege is, too precious to lose. You have shared with us the benefits of our empire, and your share extends yet further to the fear inspired in our subjects and your own security under the law. 
[4] You alone have had the freedom to be our partners in this empire, and owe it now the duty not to let it down. So show your contempt for these Corinthians--you have beaten them many times before--and for these Sicilian Greeks, none of whom was prepared even to attempt resistance when our fleet was at its prime. See them off, and make it clear that even when weakened by set backs you have the expertise to overcome any combination of strength and good fortune in the opposition.

Dio Chrysostom, The Corinthian Oration, [37.]25-27:

εἰ δέ τις οὐ Λευκανὸς ὤν, ἀλλὰ Ῥωμαῖος, οὐδὲ τοῦ πλήθους, ἀλλὰ τῶν ἱπποτρόφων, οὐδὲ τὴν φωνὴν μόνον ἀλλὰ καὶ τὴν γνώμην καὶ τὴν δίαιταν καὶ τὸ σχῆμα τῶν Ἑλλήνων ἐζηλωκώς, καὶ ταῦθ’ οὕτως ἐγκρατῶς καὶ περιφανῶς, ὡς οὔτε τῶν πρὸ αὑτοῦ Ῥωμαίων οὔτε τῶν καθ’ αὑτὸν Ἑλλήνων (εἰρήσεται γάρ) οὐδὲ εἷς· τῶν μὲν γὰρ Ἑλλήνων τοὺς ἀρίστους ἔστιν ἰδεῖν ἐκεῖσε πρὸς τὰ τῶν Ῥωμαίων πράγματα ἀποκλίνοντας, τὸν δὲ [προστάτην] πρὸς τὰ τῶν Ἑλλήνων καὶ τούτων ἕνεκα καὶ τὴν οὐσίαν καὶ τὸ πολιτικὸν ἀξίωμα καὶ πάνθ’ ἁπλῶς προϊέμενον, ἵν’ αὐτῷ περιῇ ἓν ἀντὶ πάντων Ἕλληνι δοκεῖν τε καὶ εἶναι—εἶτα τοῦτον  οὐκ ἐχρῆν παρ’ ὑμῖν ἑστάναι χαλκοῦν;  

[26] καὶ κατὰ πόλιν γε· παρ’ ὑμῖν μέν, ὅτι Ῥωμαῖος ὢν ἀφηλληνίσθη, ὥσπερ ἡ πατρὶς ἡ ὑμετέρα, παρὰ Ἀθηναίοις δέ, ὅτι ἀττικίζει τῇ φωνῇ, παρὰ Λακεδαιμονίοις δέ, ὅτι φιλογυμναστεῖ, παρὰ πᾶσι δέ, ὅτι φιλοσοφεῖ καὶ πολλοὺς μὲν ἤδη τῶν Ἑλλήνων ἐπῆρε συμφιλοσοφῆσαι αὐτῷ, οὐκ  ὀλίγους δὲ καὶ τῶν βαρβάρων ἐπεσπάσατο.  
[27] ἐπ’ αὐτὸ γὰρ τοῦτο καὶ ἐδόκει ὑπὸ τῶν θεῶν οἷον ἐξεπίτηδες κατεσκευάσθαι, Ἕλλησι μέν, ἵνα ἔχωσιν οἱ ἐπιχώριοι τῆς Ἑλλάδος παράδειγμα ὡς οὐδὲν τὸ παιδευθῆναι τοῦ φῦναι πρὸς τὸ δοκεῖν διαφέρει· Ῥωμαίοις δέ, ἵνα μηδ’ οἱ τὸ ἴδιον ἀξίωμα περιβεβλημένοι τὸ παιδεύεσθαι πρὸς τὸ ἀξίωμα παρορῶσι· Κελτοῖς δέ, ἵνα μηδὲ τῶν βαρβάρων μηδεὶς ἀπογιγνώσκῃ τῆς Ἑλληνικῆς παιδείας, βλέπων εἰς τοῦτον.

Well, if some one who is not a Lucanian but a Roman, not one of the masses but of the equestrian order, one who has affected, not merely the language, but also the thought and manners and dress of the Greeks, and that too with such mastery and manifest success as no one among either the Romans of the earlier days or the Greeks of his own time, I must say, has achieved--for while the best of the Greeks over there [in Rome] may be seen inclining to Roman ways, he inclines towards the Greek and to that end is sacrificing both his property and his political standing and absolutely everything, aiming to achieve one thing at the cost of all else, namely, not only to seem Greek but to be Greek too--taking all this into consideration, ought he not to have a bronze statue here in Corinth? 
[26] Yes, and in every city--in yours because, though Roman, he has become thoroughly hellenized, even as your own city has; in Athens because he is Athenian in his speech; in Sparta because he is devoted to athletics; in all cities everywhere because he pursues the study of wisdom and already has not only roused many of the Greeks to follow that pursuit with him but also attracted many of the barbarians. 
[27] Indeed it seesm that he has been equipped by the gods for this express purpose--for the Greeks, so that the natives of that land may have an example before them to show that culture is no whit inferior to birth with respect to reknown; for Romans, so that not even those who are wrapped up in their own self-esteem may disregard culture with respect to real esteem; for Celts, so that no one even of the barbarians may despair of attaining the culture of Greece when he looks upon this man.