VI. (5) Their view that the soul is part of the Divine being refuted.
The fifth head refers to their assertion that man's soul is part of the Divine being, and that the nature of our human state does not differ from its Creator's nature. This impious view has its source in the opinions of certain philosophers, and the Manichæans and the Catholic Faith condemns it: knowing that nothing that is made is so sublime and so supreme as that its nature should be itself God. For that which is part of Himself is Himself, and none other than the Son and Holy Spirit.
And besides this one consubstantial, eternal, and unchangeable Godhead of the most high Trinity there is nothing in all creation which, in its origin, is not created out of nothing. Besides anything that surpasses its fellow-creatures is not ipso facto God, nor, if a thing is great and wonderful, is it identical with Him “who alone does great wonders.” No man is truth, wisdom, justice; but many are partakers of truth, wisdom, and justice. But God alone is exempt from any participating: and anything which is in any degree worthily predicated of Him is not an attribute, but His very essence.
For in the Unchangeable there is nothing added, there is nothing lost: because “to be” is ever His peculiar property, and that is eternity. Whence abiding in Himself He renews all things, and receives nothing which He did not Himself give. Accordingly they are over-proud and stone-blind who, when they say the soul is part of the Divine Being, do not understand that they merely assert that God is changeable, and Himself suffers anything that may be inflicted upon His nature.
Source: Letters (New Advent)