"We that are dead to sin" (1) - In what sense?

Romans 6:2 - God forbid. We that are dead to sin, how shall we any longer live therein?

We that are dead to sin. — The meaning of this expression is very generally misunderstood, and extended to include death to the power of sin, to which it has not the smallest reference. It exclusively indicates the justification of believers, and their freedom from the guilt of sin, having no allusion to their sanctification, which, however, as the Apostle immediately proceeds to prove, necessarily follows. It was indispensable, in the view of obviating the objection proposed, distinctly to characterize both the persons, and their state of justification, to whom the answer he was about to give applied.  

Accordingly, by using the term we, he shows that he speaks of the same persons of whose justification he had been treating in the conclusion of the fourth, and in the first part of the foregoing chapter, to whom, in this way, he there refers more than twenty times. Their justification he expresses by the term dead to sin, which, though only a part of justification, implies all that it includes. No other designation could have been so well adapted to introduce the development of their state, and its inseparable consequences, as contained in the following verses. This term, then, is most appropriately employed. 

Formerly, the persons spoken of were dead in sin, but now they were dead to it, as it is said in the 7th verse, they are justified from it. In the seventh chapter, it is affirmed that believers are dead to the law. They are therefore dead to sin, for the strength of sin is the law; and consequently sin has lost its power to condemn them, their connection with it, in respect to its guilt, being for ever broken. In the 10th verse, it is said that Christ died unto sin, and liveth to God; and in the same way believers have died to sin, and are alive to God, to serve Him in newness of life. 

Romans Commentary, Robert Haldane