41 So then the one faith is, to confess the Father in the Son and the Son in the Father through the unity of an indivisible nature, not confused but inseparable, not intermingled but identical, not conjoined but coexisting, not incomplete but perfect. For there is birth not separation, there is a Son not an adoption; and He is God, not a creature. Neither is He a God of a different kind, but the Father and Son are one: for the nature was not altered by birth so as to be alien from the property of its original.
So the Apostle holds the faith of the Son abiding in the Father and the Father in the Son when he proclaims that for him there is one God the Father and one Lord Christ, since in Christ the Lord there was also God, and in God the Father there was also Lord, and They Two are that unity which is God, and They Two are also that unity which is the Lord, for reason indicates that there must be something imperfect in God unless He be Lord, and in the Lord unless He were God. And so since Both are one, and Both are implied under either name, and neither exists apart from the unity, the Apostle has not gone beyond the preaching of the Gospel in his teaching, nor does Christ when He speaks in Paul differ from the words which He spoke while abiding in the world in bodily form.
Source: On the Trinity (New Advent)