I think I have the gist of this short sentence, but I would like a little more clarity on one detail. What construction is governing the phrase φάθι εἶναι? I have parsed φάθι as the 2nd singular imperative of φημί ("to say") and εἶναι is clearly the infinitive of εἰμί ("to be").
I suppose it's possible that εἶναι is simply a complementary infinitive, complementing the imperative. Is that the case? Had Plato omitted the imperative, would εἶναι still be infinitive, or would it be conjugated instead?1
τοῦτο τοίνυν τὸ τὴν ἀλήθειαν παρέχον τοῖς γιγνωσκομένοις καὶ τῷ γιγνώσκοντι τὴν δύναμιν ἀποδιδὸν τὴν τοῦ ἀγαθοῦ ἰδέαν φάθι εἶναι:
This, then, which furnishes truth to the objects of knowledge and gives power to the knower, you must say is the idea of the good.
1 At first I suspected an ACI, but the imperative threw me off — I'm not accustomed to seeing this informal 2nd sg imperative in the middle of an ACI.