23 You hear the words, I and the Father are one. Why do you rend and tear the Son away from the Father? They are a unity: an absolute Existence having all things in perfect communion with that absolute Existence, from Whom He is. When you hear the Son saying, I and the Father are one, adjust your view of facts to the Persons; accept the statement which Begetter and Begotten make concerning Themselves. Believe that They are One, even as They are also Begetter and Begotten.
Why deny the common nature? Why impugn the true Divinity? You hear again, The Father in Me, and I in the Father. That this is true of Father and of Son is demonstrated by the Son's works. Our science cannot envelope body in body, or pour one into another, as water into wine; but we confess that in Both is equivalence of power and fullness of the Godhead. For the Son has received all things from the Father; He is the Likeness of God, the Image of His substance. The words, Image of His substance, discriminate between Christ and Him from Whom He is, but only to establish Their distinct existence, not to teach a difference of nature; and the meaning of Father in Son and Son in Father is that there is the perfect fullness of the Godhead in Both.
The Father is not impaired by the Son's existence, nor is the Son a mutilated fragment of the Father. An image implies its original; likeness is a relative term. Now nothing can be like God unless it have its source in Him; a perfect likeness can be reflected only from that which it represents; an accurate resemblance forbids the assumption of any element of difference. Disturb not this likeness; make no separation where truth shows no variance, for He Who said, Let us make man after our image and likeness, by those words Our likeness revealed the existence of Beings, Each like the Other.
Touch not handle not, pervert not. Hold fast the Names which teach the truth, hold fast the Son's declaration of Himself. I would not have you flatter the Son with praises of your own invention; it is well with you if you be satisfied with the written word.
Source: On the Trinity (New Advent)