Beatitude is the perfect good of the intellectual nature, to which it pertains to know the sufficiency of the good it possesses and to have dominion over its own operations; and since this is eminently proper to God, so also is Beatitude; for those things which exist in creatures separately and by way of composition, pre-exist in Him in simplicity and unity.
This Beatitude is to be attributed to God and to the other Blessed according to the intellect, because it is the perfect good of the intellectual nature. Now the most perfect operation of such a nature is that according to which, in certain manner, it knows all; and since to be and to know is, in reality, the same thing in God, intellectual Beatitude is to be attributed to Him; so also to the Blessed, who are called so by assimilation to the Divine Beatitude.
If beatitude be considered on the part of its object, then God alone is Beatitude; for beatitude consists in possessing the knowledge of God; but if we consider the act of him who knows, beatitude is something created in the beatified creature. And whatever is desirable in any beatitude pre-exists eminently in that of God, which consists in a continuous and most certain contemplation of Himself and of all others, and in the active government of the entire universe. For riches it has every kind of sufficiency, for power omnipotence, for dignity universal rule, for fame universal admiration.
- text taken from Compendium of the Summa Theologica of Saint Thomas Aquinas, by Bishop Berardus Bongiovanni