16 Would you receive yet a third testimony to Christ's Godhead? Hear Esaias saying, Egypt has laboured, and the merchandise of Ethiopia: and soon after, In You shall they make supplication, because God is in You, and there is no God save You. For You are God, and we knew it not, the God of Israel, the Saviour. You see that the Son is God, having in Himself God the Father: saying almost the very same which He has said in the Gospels: The Father is in Me, and I am in the Father. He says not, I am the Father, but the Father is in Me, and I am in the Father.
And again He said not, I and the Father am one, but, I and the Father am one, that we should neither separate them, nor make a confusion of Son-Father. One they are because of the dignity pertaining to the Godhead, since God begot God. One in respect of their kingdom; for the Father reigns not over these, and the Son over those, lifting Himself up against His Father like Absalom: but the kingdom of the Father is likewise the kingdom of the Son. One they are, because there is no discord nor division between them: for what things the Father wills, the Son wills the same.
One, because the creative works of Christ are no other than the Father's; for the creation of all things is one, the Father having made them through the Son: For He spoke, and they were made; He commanded, and they were created, says the Psalmist. For He who speaks, speaks to one who hears: and He who commands, gives His commandment to one who is present with Him.
Source: Catechetical Lectures (New Advent)