That no Created Substance can of its natural power arrive to see God as He essentially is

THE property of a higher nature cannot be attained by a lower nature except by the action of that higher nature to which it properly belongs. But to see God by the divine essence is the property of the divine nature: for it is proper to every agent to act by its own proper form. Therefore no subsistent intelligence can see God by the divine essence except through the action of God bringing it about.

5. To see the substance of God transcends the limits of every created nature: for it is proper to every intelligent created nature to understand according to the mode of its substance: but the divine substance is not intelligible according to the mode of any created substance (Chap. XLIX).

Hence it is said: The grace of God is life everlasting (Rom. vi, 23). For we have shown that the happiness of man consists in the vision of God, which is called life everlasting, whereunto we are led solely by the grace of God, because such vision exceeds the faculty of every creature, and it is impossible to attain it except by an endowment from God. And the Lord says: I will manifest myself to him (John xiv, 21).

3.51 : How God is seen as He essentially is
3.53 : That a Created Intelligence needs some influx of Divine Light to see God in His Essence