The Son as radiance and the lesson of obedience
The author is explaining Hebrews 1:3 and 5:8, showing how the Son is called radiance to teach his coeternity with the Father, and how Christ's suffering displayed the lesson of obedience.
He calls the Father “glory” in a reverent way, and the Son “radiance,” teaching by this example that the Son is from the Father and coeternal with the Father. For radiance is like that: it comes from the sun and coexists with it. So no one could ever say that the sun is older than its own radiance. And he said this without the syllable “Father,” following the idiom of Scripture.
In place of “Though he was a son, he learned obedience through what he suffered,” it means that through suffering he had the lesson of obedience and displayed it most perfectly, even though he was Son. We too are accustomed to speak this way.
Greek
{Hebr 1,3} ∆όξαν μὲν εὐφήμως κέκληκε
τὸν πατέρα, ἀπαύγασμα δὲ τὸν υἱόν, διδάσκων ἐκ παραδείγματος ἐκ πατρός τε αὐτὸν
εἶναι καὶ συναΐδιον τῷ πατρί· τοιοῦτον γὰρ τὸ ἀπαύγασμα ἐκ τοῦ ἡλίου τε ὂν καὶ
συνυπάρχον αὐτῷ. οὐκοῦν ἂν ἥλιον εἴποι τις οὔποτε τοῦ οἰκείου πρεσβύτερον
ἀπαυγάσματος. ἔφη δὲ αὐτὸ δίχα τῆς πατρὸς συλλαβῆς κατὰ τὸ ἰδίωμα τῆς γραφῆς.
{Hebr 5,8} Ἀντὶ τοῦ τῷ πάθει τὸ μάθημα τῆς ὑπακοῆς ἔχων ἐντελέστατον
ἐπεδείξατο, καὶ ταῦτα τυγχάνων υἱός. οὕτω δὲ καὶ ἡμεῖς λέγειν εἰώθαμεν.