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In creatures, relations are extrinsic to the essence of the creature. For example, my essence* is humanness, but my relation to my father, mother, wife, friends, or even to God is not humanness; it's not my essence. However, in the Holy Trinity, the Divine Relations are the essence of God Himself. This is a consequence of the Trinity being supremely simple.
*Essence or quiddity is the answer to the question: "What is it?" ("Quid est?")
St. Thomas Aquinas writes, answering the question "Whether relation in God is the same as His essence?,"
…in God relation and essence do not differ from each other, but are one and the same.
cf. Fr. Gilles Emery, O.P.'s The Trinitarian Theology of St. Thomas Aquinas ch. 5 "Relations" (p. 78 f.)
Thus, it is clear that since God is infinitely perfect, and He is His Trinitarian relations, these relations are infinitely perfect.
When we behold the Beatific Vision in heaven, we do not become God; we maintain our individuality. We will know God as perfectly as His grace enables us us know him,* but not as perfectly as He knows Himself.
*There are hierarchies in heaven; not all saints (i.e., those in heaven) are equally close to God. The Blessed Virgin Mother is closest in her relation to her Son.
St. Thomas Aquinas writes in his Compendium Theologiæ cap. 106 ("Fruition of Natural Desire in the Beatific Vision [in Heaven]"):
…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.
Also, the classic work on how there are hierarchies in heaven is Dionysius the Areopagite's On the Heavenly Hierarchy.