In order to behold God directly, the soul requires the light of glory.
The human soul is, by nature, not capable of seeing God “face to face.”
For this, it needs a special light: the so-called “light of glory” (lumen gloriae).
This “light of glory” is a supernatural elevation of the intellect, through which the soul becomes able to behold God directly, without perishing or falling into error.
Without this light, the Church says, man could neither endure nor comprehend God.
The lumen gloriae is therefore a gift of grace, bestowed upon the soul when it enters into the eternal vision of God.
Source (Latin):
Benedict XII, Constitutio dogmatica “Benedictus Deus” (January 29, 1336), in: Denzinger-Schönmetzer, DS 1003
“…that blessed vision of the divine essence does not occur without the illumination by divine power and the light of glory, which is adapted to the highest capacity of the intellect, elevating it so that it may be capable of the vision of the divine essence.”
Source (Systematic):
DS 1003 / CCC No. 1028