This is a saying attributed to Jesus from the Gospel of Thomas, a new translation of Saying 61:
"Therefore I say: If someone becomes like God, he will become full of light. But if he becomes one, separated from God, he will be full of darkness." ("The Fifth Gospel," Patterson, Robinson, Bethge, Trinity Press International)
What kind of "Light" is this Saying talking about? In my view the Gospel of Thomas is in the tradition of contemplative or mystical Christianity and the "Light" of Thomas referred to here is not about metaphor but real Light seen -- an actual seeing of Light with spiritual vision -- the Light of God.
The great Egyptian saint Evagrius described that, for him, often his time of prayer was "interrupted" by the manifestation of "the Holy Light of the Trinity." He spoke of reaching a mystical level where it was no longer necessary to pray, because the Light of God engulfed his vision. In a stage of contemplative prayer that he called "Pure Prayer," Evagrius says, "Prayer ceases, and one becomes astonished, is caught up in wonder at the Light of God." "The person who has entered the Place of the Mysteries remains in wonder at them, and this is the true prayer which opens the Door to the Treasures of God." ("The Syriac Fathers On Prayer and the Spiritual Life")
