It is actually one of three items required to enter the inner sanctorum of the mask, however by the time you reach there you already have it. If you try to get to the door via cheats, the Archon will tell you you need the all three items including the armor.. The 'Armor of Light' is an allusion to the 'armor of light' mentioned in Romans: 13:12, and the Armor of God mentioned in Ephesians 6:10-18.