26 Now we have of necessity set these things forth with this object, that in whatever direction the deception of heretics betakes itself, it might yet be kept in check by the boundaries and limits of the gospel truth. For Christ dwells in us, and where Christ dwells God dwells. And when the Spirit of Christ dwells in us, this indwelling means not that any other Spirit dwells in us than the Spirit of God. But if it is understood that Christ dwells in us through the Holy Spirit, we must yet recognise this Spirit of God as also the Spirit of Christ.
And since the nature dwells in us as the nature of one substantive Being, we must regard the nature of the Son as identical with that of the Father, since the Holy Spirit Who is both the Spirit of Christ and the Spirit of God is proved to be a Being of one nature. I ask now, therefore, how can They fail to be one by nature? The Spirit of Truth proceeds from the Father, He is sent by the Son and receives from the Son. But all things that the Father has are the Son's, and for this cause He Who receives from Him is the Spirit of God but at the same time the Spirit of Christ.
The Spirit is a Being of the nature of the Son but the same Being is of the nature of the Father. He is the Spirit of Him Who raised Christ from the dead; but this is no other than the Spirit of Christ Who was so raised. The nature of Christ and of God must differ in some respect so as not to be the same, if it can be shown that the Spirit which is of God is not the Spirit of Christ also.
Source: On the Trinity (New Advent)