A Catholic about to state that the substance of the Father and the Son is one, must not begin at that point: nor hold this word all important as though true faith did not exist where the word was not used. He will be safe in asserting the one substance if he has first said that the Father is unbegotten, that the Son is born, that He draws His personal subsistence from the Father in might, honour and nature, that He is subject to the Father as to the Author of His being, that He did not commit robbery by making Himself equal with God, in whose form He remained, that He was obedient unto death. He did not spring from nothing, but was born. He is not incapable of birth but equally eternal. He is not the Father, but the Son begotten of Him. He is not any portion of God, but is whole God. he is not Himself the source but the image; the image of God born of God to be God. He is not a creature but is God. Not another God in the kind of His substance, but the one God in virtue of the essence of His exactly similar substance. God is not one in Person but in nature, for the Born and the Begetter have nothing different or unlike. After saying all this, he does not err in declaring one substance of the Father and the Son. Nay, if he now denies the one substance he sins.
~Hilary of Poiters On the Councils 69