Equality must be postulated of the Divinity, in Whom there exists nothing of more or less; for unequal quantities are not one; and as quantity, in the Divinity, means nothing else than Essence, without equality there could not be One Essence, so neither would there be Three Persons and One God.
Hence the Son is coeternal with the Father, Who does not generate Him by His Will, but by His Nature, which is Eternally Perfect. Nor is the Act by which the Son is produced successive, for this Generation is not material, neither has it the nature of motion; but the Son was when the Father was; and so of the Holy Ghost. With created agents it may happen that a thing is posterior to its principle because the power of nature to act is not always perfect, but comes by degrees; as man has not always power to generate. It is also due to action that created things are not simultaneous with their principle; for action is successive in creatures; but it is not so in God.
And in the Divine Persons there is Order of Nature; for there is always order where there is comparison with some principle or single cause: but in the Divinity Principle bears relation to Origin, without priority. Thus Augustine calls that Order of Nature where one is from another, not where one precedes the other.
The Son participates perfectly in the Nature of the Father, in which there is no defect; nor is there succession in generation, nor transmutation such as is found in creatures, which pass from the potential to the actual and reach equality by increase: therefore we say that the Son is equal in greatness to the Father, for the greatness of God is the perfection of His Nature; but the Father is considered as giving, and the Son as receiving.
And the Son is in the Father, and conversely, according to Essence, Relation and Origin, which Three are found in the Divinity. The Father is in the Son by His Essence, which He communicates without any transmutation of Himself; from which it follows that the Essence of the Father being in the Son, the Father Himself is in the Son, and conversely. Likewise, according to Relation, it follows that One of the relative opposites is in the Other.
With regard to Origin, it is evident that the Procession of the Word is not intelligible to anything without, but remains in the speaker, and that which is expressed by the Word remains in the Word; for the going forth of the Son from the Father is according to the mode of Interior Procession, as a word which comes from the heart and remains in it.
Equality of power belongs to the Son because He participates fully in the Nature of the Father; for we see, even in creatures, that power in action is in proportion to the perfection of anything. Hence the Father, in communicating His Essence, communicates also His Power and Wisdom.
- text taken from Compendium of the Summa Theologica of Saint Thomas Aquinas, by Bishop Berardus Bongiovanni