They have to know who they are and how they are situated in relation to Ultimate Reality.
"Whoso recognizes himself recognizes his Lord."
In order to recognize God as Light, they need to recognize themselves as light.
In a famous prayer, the Prophet said,
O God, place in my heart a light, in my hearing a light, in my eyesight a light, on my right hand a light, on my left hand a light, before me a light, behind me a light, above me a light, below me a light, and make me into a light.
In this prayer, the Prophet is asking God to show him the light that he possesses in himself because he was created by the Supreme Light. Only when we find light in ourselves can we recognize God as Light.