Objection 1. It seems that God is the beatitude of each of the blessed. For God is the supreme good, as was said above (6, 2 and 4). But it is quite impossible that there should be many supreme goods, as also is clear from what has been said above (Question 11, Article 3). Therefore, since it is of the essence of beatitude that it should be the supreme good, it seems that beatitude is nothing else but God Himself.
Objection 2. Further, beatitude is the last end of the rational nature. But to be the last end of the rational nature belongs only to God. Therefore the beatitude of every blessed is God alone.
On the contrary, The beatitude of one is greater than that of another, according to 1 Corinthians 15:41: "Star differeth from star in glory." But nothing is greater than God. Therefore beatitude is something different from God.
I answer that, The beatitude of an intellectual nature consists in an act of the intellect. In this we may consider two things, namely, the object of the act, which is the thing understood; and the act itself which is to understand. If, then, beatitude be considered on the side of the object, God is the only beatitude; for everyone is blessed from this sole fact, that he understands God, in accordance with the saying of Augustine (Confess. v, 4): "Blessed is he who knoweth Thee, though he know nought else." But as regards the act of understanding, beatitude is a created thing in beatified creatures; but in God, even in this way, it is an uncreated thing.
Reply to Objection 1. Beatitude, as regards its object, is the supreme good absolutely, but as regards its act, in beatified creatures it is their supreme good, not absolutely, but in that kind of goods which a creature can participate.
Reply to Objection 2. End is twofold, namely, "objective" and "subjective," as the Philosopher says (Greater Ethics i, 3), namely, the "thing itself" and "its use." Thus to a miser the end is money, and its acquisition. Accordingly God is indeed the last end of a rational creature, as the thing itself; but created beatitude is the end, as the use, or rather fruition, of the thing.
Source: Summa Theologica (New Advent)