Thomas Aquinas, Compendium of Theology

Once this end is reached, natural desire must find its full fruition. The divine essence thus united to the intellect of the one who sees God, is the adequate principle for knowing everything, and is the source of all good, so that nothing can remain to be desired. This, too, is the most perfect way of attaining likeness with God: to know God in the way He knows Himself, by His own essence.

Of course, we shall never comprehend Him as He comprehends Himself. This does not mean that we shall be unaware of some part of Him, for He has no parts. It means that we shall not know Him as perfectly as He can be known, since the capacity of our intellect for knowing cannot equal His truth, and so cannot exhaust His knowability. God's knowability or truth is infinite, whereas our intellect is finite. But His intellect is infinite, just as His truth is; and so He alone knows Himself to the full extent that He is knowable; just as a person comprehends a demonstrable conclusion if he knows it through demonstration, but not if he knows it only in an imperfect way, on merely probable grounds.

This ultimate end of man we call beatitude. For a man's happiness or beatitude consists in the vision whereby he sees God in His essence. Of course, man is far below God in the perfection of his beatitude. For God has this beatitude by His very nature, whereas man attains beatitude by being admitted to a share in the divine light, as we said in the previous chapter.