Lost quotation

There's an ancient author who said something like "Nowadays we consider someone a Hellene if they share our culture rather than out ancestors." I've lost my note of this.   Was it Isocrates?

You're right. It is Isocrates who said:
Tοσοῦτον δ' ἀπολέλοιπεν ἡ πόλις ἡμῶν περὶ τὸ φρονεῖν καὶ λέγειν τοὺς ἄλλους ἀνθρώπους, ὥσθ' οἱ ταύτης μαθηταὶ τῶν ἄλλων διδάσκαλοι γεγόνασι, καὶ τὸ τῶν Ἑλλήνων ὄνομα πεποίηκε μηκέτι τοῦ γένους ἀλλὰ τῆς διανοίας δοκεῖν εἶναι, καὶ μᾶλλον Ἕλληνας καλεῖσθαι τοὺς τῆς παιδεύσεως τῆς ἡμετέρας ἢ τοὺς τῆς κοινῆς φύσεως μετέχοντας.
Our city has so far surpassed other men in thought and speech that students of Athens have become the teachers of others, and the city has made the name “Greek” seem to be not that of a people but of a way of thinking; and people are called Greeks because they share in our education (paideusis) rather than in our birth.
Thanks for mentioning it.

Thank you for that. Can you say exactly where in his speeches it comes?

That book is not easy to find. It is from the Panegyricus section 50.
It is available on www.perseus.tufts.edu/

Thank you! The Loeb edition is available as pdf, but I didn't fancy searching three volumes; I have enough stuff waiting to be read.