Leithart describes the four primary paradigms or images that Athanasius finds in Scripture to defend the eternality and full deity of the Son. Here is the first:
"The point of the radiance image (in Heb 1:3) is to emphasize that the Son is necessary to the Father's existence, as necessary as the rays of light are to the light source, and therefore the Son is just as eternal as the Father. Just as there is no light that does not radiate, so the Father is not without the radiance of his glory. When one reads of the Son as the "radiance" of the Father's glory, "who has so little sense as to doubt of the eternity of the Son? For when did man see light without the brightness of its radiance, that he may say of the Son, 'There was once, and He was not,' or 'Before His generation He was not'?" (Discourses 1.12)...The light-radiance imagery not only proves the co-eternity of the Father and the Son but also shows, in Athanasius's view, that the Arian position is nonsensical...The Father cannot create without his Son, any more than a light can illuminate without its radiance." - Leithart, Athanasius, pp44-45.