When it is said
The priest says it quietly at every Divine Liturgy, while the Alleluia is sung before the Gospel. It stands in the Liturgy of St John Chrysostom and in the Liturgy of St Basil the Great.
The same prayer, a little changed, is one of the morning prayers the priest says quietly at Matins, the morning service.
It is also said at home before reading the Bible. The Orthodox Church in America gives it as its prayer before reading Holy Scripture, and the Slavonic book of Psalms prints it at the end of its eleventh section.
Where it comes from
No author known
Is it St John Chrysostom’s prayer?
Not as far as anyone knows. It is said in the Liturgy named for St John Chrysostom, archbishop of Constantinople around AD 400. But no one knows who wrote it. The prayer that Russian prayer books give as his, for reading Scripture, is a different one. It begins: “O Lord Jesus Christ, open the ears of my heart to hear Your word.”
Hieromonk Job (Gumerov), Pravoslavie.ru: which prayer to say before reading the Gospel
In the Liturgy of St James
The same prayer is said before the Gospel in the Liturgy of St James, named for James the brother of the Lord, the first bishop of Jerusalem. There it asks for “the pure light of knowledge”. It ends in its own way: “For You are the good news and the light, Saviour and guardian of our souls and bodies.”
The Divine Liturgy of St James in Greek (Greek Orthodox Archdiocese of America)
Also a morning prayer
At Matins six psalms are read near the start. After the first three, the priest quietly says a set of morning prayers. The ninth of them is this prayer.
J. N. W. B. Robertson, The Divine Liturgies (1894): the Orthros
The Scripture in it
For God, Who said, “Let light shine out of darkness,” made His light shine in our hearts to give us the light of the knowledge of the glory of God in the face of Jesus Christ.
I ask that the eyes of your heart may be enlightened, so that you may know the hope of His calling, the riches of His glorious inheritance in the saints …
Then He opened their minds to understand the Scriptures.
So I say, walk by the Spirit, and you will not gratify the desires of the flesh.
Bible text: Berean Standard Bible, public domain.
In Greek
Ἔλλαμψον ἐν ταῖς καρδίαις ἡμῶν, φιλάνθρωπε Δέσποτα, τὸ τῆς Σῆς θεογνωσίας ἀκήρατον φῶς, καὶ τοὺς τῆς διανοίας ἡμῶν ὀφθαλμοὺς διάνοιξον εἰς τὴν τῶν εὐαγγελικῶν Σου κηρυγμάτων κατανόησιν.
Ἔνθες ἡμῖν καὶ τὸν τῶν μακαρίων Σου ἐντολῶν φόβον, ἵνα τὰς σαρκικὰς ἐπιθυμίας πάσας καταπατήσαντες, πνευματικὴν πολιτείαν μετέλθωμεν, πάντα τὰ πρὸς εὐαρέστησιν τὴν Σὴν καὶ φρονοῦντες καὶ πράττοντες.
Σὺ γὰρ εἶ ὁ φωτισμὸς τῶν ψυχῶν καὶ τῶν σωμάτων ἡμῶν, Χριστὲ ὁ Θεός, καὶ Σοὶ τὴν δόξαν ἀναπέμπομεν, σὺν τῷ ἀνάρχῳ Σου Πατρὶ καὶ τῷ παναγίῳ καὶ ἀγαθῷ καὶ ζωοποιῷ Σου Πνεύματι, νῦν καὶ ἀεὶ καὶ εἰς τοὺς αἰῶνας τῶν αἰώνων. Ἀμήν.