Because the God who said, “Light shall shine out of darkness,” is the one who has shone in our hearts for the illumination of the knowledge of the glory of God in the face of Jesus Christ.
Causal Conjunction and Thematic Emphasis: ὅτι ὁ Θεὸς– ὅτι: Conjunction introducing a causal or explanatory clause—”because” or “for.” – ὁ Θεός: Nominative singular subject—”God.” – This phrase links directly with the preceding verse (2 Corinthians 4:5), explaining why Paul and his companions preach Christ as Lord and not themselves.… Learn Koine Greek