Do Not Let Sin Reign
The like exhortation the Apostle gives, Let not sin reign in your mortal bodies, that you should obey it in the lusts thereof. It is true that sin will remain in us, but it must not reign over us, for if grace reign in your hearts, then sin must not reign – grace will have no competitor.
We read of Alexander the Great, that he was never content till he had conquered all the world; so it is with grace, it is never at rest till it have conquered this little world, this body of sin, and put down every high thought that does exalt itself against God.
This exhortation is urged more plainly. Mortify your members that are on the earth, fornication, uncleanness, inordinate desires, affections & et cetera. As before, sin is compared to a body, so here, particular sins are compared to so many members of that body, that as all the member do work together for the preservation of the whole, so does every least conduct and concur to the preservation and continuation of this body of sin.
Mortification as External
For the more distinct handling of the point, consider that mortification may be distinguished as something that is external and is without us [outside of us], or else as something that is internal and is down within us: As it is external it is said to be legal mortification, when as a man is dead in law, as a malefactor is said to be a dead man when he is condemned.
So sin is then said to be dead when it is forgiven and God is said to send his Son in the likeness of sinful flesh and for sin condemned sin in the flesh, that is, that sin should be certainly put to death (Rom. 8.3). As it was certain that Christ had taken our nature on him, a great comfort it is to a distressed soul that does cast itself upon the Lord Jesus Christ: though at first it doe not apprehend the power of sin mortified, yet apprehending our plenteous redemption purchased by Christ, he concludes, that sin shall certainly die, because Christ has already condemned it.
And as David said when Jonathon persuaded him to the contrary, As the Lord lives, and as your soul lives, there is but a step between me and death (1 Sam. 20:3). So whatsoever conceit [thought] the foolish heart of man may have, that his lusts are so strong, as there is not possibility of subduing them, yet for as much as the matter is now in the hand of Christ, who has past sentence on them, it is certain that there is but a step between them and death.