Objection 1. It seems that God is not called blessed in respect to His intellect. For beatitude is the highest good. But good is said to be in God in regard to His essence, because good has reference to being which is according to essence, according to Boethius (De Hebdom.). Therefore beatitude also is said to be in God in regard to His essence, and not to His intellect.
Objection 2. Further, Beatitude implies the notion of end. Now the end is the object of the will, as also is the good. Therefore beatitude is said to be in God with reference to His will, and not with reference to His intellect.
On the contrary, Gregory says (Moral. xxxii, 7): "He is in glory, Who whilst He rejoices in Himself, needs not further praise." To be in glory, however, is the same as to be blessed. Therefore, since we enjoy God in respect to our intellect, because "vision is the whole of the reward," as Augustine says (De Civ. Dei xxii), it would seem that beatitude is said to be in God in respect of His intellect.
I answer that, Beatitude, as stated above (Article 1), is the perfect good of an intellectual nature. Thus it is that, as everything desires the perfection of its nature, intellectual nature desires naturally to be happy. Now that which is most perfect in any intellectual nature is the intellectual operation, by which in some sense it grasps everything. Whence the beatitude of every intellectual nature consists in understanding. Now in God, to be and to understand are one and the same thing; differing only in the manner of our understanding them. Beatitude must therefore be assigned to God in respect of His intellect; as also to the blessed, who are called blesses [beati] by reason of the assimilation to His beatitude.
Reply to Objection 1. This argument proves that beatitude belongs to God; not that beatitude pertains essentially to Him under the aspect of His essence; but rather under the aspect of His intellect.
Reply to Objection 2. Since beatitude is a good, it is the object of the will; now the object is understood as prior to the act of a power. Whence in our manner of understanding, divine beatitude precedes the act of the will at rest in it. This cannot be other than the act of the intellect; and thus beatitude is to be found in an act of the intellect.
Source: Summa Theologica (New Advent)