Here are two different translations of the final bit of Proclus’ Essay 5 (K69.10-19), on the Republic.
What I find interesting in the translation choices — not knowing Ancient Greek — is the word choice between sin and failing. It’s also interesting how the first translation separates out the hymns for the Gods from those for humans, whereas the latter puts “with or without myths” in a clause that makes it seem as if it applies to both.
Baltzly et al.:
Thus, if it is necessary for me to go on about things that ought not be expressed (ta anexoista), then it is obvious who this poet is: he moves the Sirens to sing, ‘uttering a single voice, one tone’ as Plato says in the myth in the tenth book of the Republic (617b). As Timaeus says, it is he who sets in motion the orbits of the divine souls that are turned with rhythmic motions in proportion. Everything that has its origin from souls is the work (poiêma) of Apollo, harmonious and rhythmic. Looking to this [universal poet] let the earthly poet compose hymns to the gods. And let him compose hymns to good men, whether in myth or without myth. Otherwise, if he turns to other subject matter, let him know that he sins against poetry and against Apollo.
And so, if I must express what should not be divulged, it is clear who the poet is. He moves the Sirens to sing “casting a single voice, a single tone” as the myth [of Er] in the tenth book of the Republic says [617b]. As Timaeus says [Tim. 36c], he sets in motion the cycles of the divine souls, to revolve rhythmically and in an orderly manner — and all things that have souls at their origin are the creations (ποιήματα) of Apollo, harmonious and rhythmic. Looking to him, let the poet of this world celebrate the gods and let him celebrate good men, with or without myths. Alternatively, should he turn to other things, let him recognize that he is failing with respect both to poetry and to Apollo.
When I pray to Apollon, I often wonder at the responsibility of composing stories and poems in a way that does justice to the Gods. Hieropoeia, hieropoeia, hieropoeia.