Commenting on Hebrews 1:3, which says that the Son is ‘the radiance of God’s glory and the exact representation of his being’, the fourth-century theologian, Gregory of Nyssa, explained that: as the light from the lamp is of the nature of that which sheds the brightness, and is united with it (for as soon as the lamp appears the light that comes from it shines out simultaneously), so in this place the Apostle would have us consider both that the Son is of the Father, and that the Father is never without the Son; for it is impossible that glory should be without radiance, as it is impossible
...more

