The only knowledge of God that philosophers can attain is a knowledge based on God’s effects in our world. They can prove, as Aquinas does in his ‘five ways,’ that there is a universal cause, God; that they can give an answer to the question whether He exists. But they cannot give anything like a full account of what God is; knowledge of the divine essence remains hidden to human beings. In this restricted philosophical knowledge, however, our desire to know is not satisfied, for we retain by nature to know the essence of God.
Aquinas argues that our perfect happiness, the fulfillment of our natural desire, can consist only in the contemplation of God’s essence, in the vision of God (visio Dei), in which we see the answer to the question what he is. From this he draws the conclusion that our “complete happiness cannot consist in theoretical knowledge,” that is, in philosophy, broadly conceived. The vision of God surpasses our natural powers and capacities. This end of ours is literally supernatural… The impossibility of the vision of God is also contrary to reason, because human happiness is that in which human desire comes to rest.
Now it is our natural desire, when we see an effect, to inquire into its cause. This desire will not come to rest until we reach the first cause, namely the divine essence itself. “Therefore God will be seen in his essence.”
Jan A. Aertsen, from the Cambridge Companion to Aquinas (Cambridge University Press: 1993) pp. 32-33