John Chrysostom

Again, what is good he applies to them, and the distressing part was short and parenthetic. And that which is an object of desire, is on either side of it, and put at length too, so as to throw the other into shade. Now this he says, not as affirming that the Spirit is Christ, far from it, but to show that he who hath the Spirit not only is called Christ's, but even hath Christ Himself. For it cannot but be that where the Spirit is, there Christ is also. …

Homily on Romans 13 ↗

Theophylact of Ohrid

Again he comforts the listeners when he says, "but if Christ is in you." Some understood Christ here to mean the Spirit; but this is incorrect. The Apostle gives us to understand that he who has the Spirit has Christ Himself within him; for where one of the Persons of the Holy Trinity is, there also are the other Persons. What then will happen if Christ is in us? Then the body is dead with respect to sin, and the Holy Spirit in you is life, that is, He not only lives Himself, but can also impart life to others. …

Commentary on Romans ↗

Irenaeus

Rightly therefore does the apostle declare, "Flesh and blood cannot inherit the kingdom of God;" and, "Those who are in the flesh cannot please God:" not repudiating [by these words] the substance of flesh, but showing that into it the Spirit must be infused. …

Irenaeus Against Heresies Book 5 ↗

Augustine of Hippo

Paul shows that both life and death exist in a man living in his body—death in his body, life in his spirit.

City of God 20.15 ↗

Thomas Aquinas

Then when he says, "but if Christ," he shows that through the grace of Christ or through the Holy Spirit we are freed from punishment. And first, he shows that we are freed by the Holy Spirit in the future from bodily death; second, that in the meantime the Holy Spirit helps us against the infirmities of the present life, at "likewise, the Spirit" (Rom 8:26). Concerning the first he does three things. …

Commentary on Romans ↗