Objection 1. It seems that beatitude does not belong to God. For beatitude according to Boethius (De Consol. iv) "is a state made perfect by the aggregation of all good things." But the aggregation of goods has no place in God; nor has composition. Therefore beatitude does not belong to God.
Objection 2. Further, beatitude or happiness is the reward of virtue, according to the Philosopher (Ethic. i, 9). But reward does not apply to God; as neither does merit. Therefore neither does beatitude.
On the contrary, The Apostle says: "Which in His times He shall show, who is the Blessed and only Almighty, the King of Kings and Lord of Lords." (1 Timothy 6:15).
I answer that, Beatitude belongs to God in a very special manner. For nothing else is understood to be meant by the term beatitude than the perfect good of an intellectual nature; which is capable of knowing that it has a sufficiency of the good which it possesses, to which it is competent that good or ill may befall, and which can control its own actions. All of these things belong in a most excellent manner to God, namely, to be perfect, and to possess intelligence. Whence beatitude belongs to God in the highest degree.
Reply to Objection 1. Aggregation of good is in God, after the manner not of composition, but of simplicity; for those things which in creatures is manifold, pre-exist in God, as was said above (4, 2; 13, 4), in simplicity and unity.
Reply to Objection 2. It belongs as an accident to beatitude or happiness to be the reward of virtue, so far as anyone attains to beatitude; even as to be the term of generation belongs accidentally to a being, so far as it passes from potentiality to act. As, then, God has being, though not begotten; so He has beatitude, although not acquired by merit.
Source: Summa Theologica (New Advent)