27 But you, heretic, as you wildly rave and are driven about by the Spirit of your deadly doctrine the Apostle seizes and constrains, establishing Christ for us as the foundation of our faith, being well aware also of that saying of our Lord, If a man love Me, he will also keep My word; and My Father will love him, and We will come unto him, and make Our abode with him. For by this He testified that while the Spirit of Christ abides in us the Spirit of God abides in us, and that the Spirit of Him that was raised from the dead differs not from the Spirit of Him that raised Him from the dead.
For they come and dwell in us: and I ask whether they will come as aliens associated together and make Their abode, or in unity of nature? Nay, the teacher of the Gentiles contends that it is not two Spirits— the Spirits of God and of Christ— that are present in those who believe, but the Spirit of Christ which is also the Spirit of God. This is no joint indwelling, it is one indwelling: yet an indwelling under the mysterious semblance of a joint indwelling, for it is not the case that two Spirits indwell, nor is one that indwells different from the other.
For there is in us the Spirit of God and there is also in us the Spirit of Christ, and when the Spirit of Christ is in us there is also in us the Spirit of God. And so since what is of God is also of Christ, and what is of Christ is also of God, Christ cannot be anything different from what God is. Christ, therefore, is God, one Spirit with God.
Source: On the Trinity (New Advent)