Let not sin therefore reign in your mortal body, that ye should obey the lusts thereof. (The reading of the Textus Receptus, "obey it in the lusts thereof," has but weak support.) Though our "old man" is conceived of as crucified with Christ—though this is theoretically and potentially our position—yet our actual lives may be at variance with it; for we are still in our present "mortal body," with its lusts remaining; and sin is still a power, not yet destroyed, which may, if we let it, have domination over us still.
Regeneration is not regarded as having changed our nature, or eradicated all our evil propensions, but as having introduced into us a higher power—"the power of his resurrection" (Philippians 3:10)—in virtue of which we may resist the attempted domination of sin.
But it still rests with us whether we will give our allegiance to sin or to Christ. οὐ γὰρ τὴν φύσιν ἦλθεν ἀνελεῖν ἀλλὰ τὴν προαίρεσιν διορθῶσαι (Chrysostom). The lusts, obedience to which is equivalent to letting sin reign, are said to be those of our "mortal body," because it is in our present bodily organization that the lusts tempting us to evil rise.
But it is not in their soliciting us, but in the will assenting to them, that the sin lies. "Quia non consentimus desideriis pravis in gratia sumus". "Cupiditates corporis sunt fomes, peccatum ignis" (Bengel).
The epithet θνητῷ ("mortal'') is fitly used as distinguishing our present perishable framework—the earthen vessels in which we have our treasure (2 Corinthians 4:7)—from our real inward personality, ἔσωθεν ἄνθρωπος (2 Corinthians 4:16), which is regarded as having risen with Christ, so as to live to God for ever.
"Vos enim, viventes, abalienati estis a corpore vestro (cf. Romans 8:10)" (Bengel).