Romans 6:12 Therefore do not let sin reign in your mortal body so that you obey its evil desires.

Sin is not just lust, greed, hate, adultery. Those are sins! Behaviors. Actions. Out workings of sin called sins. Sin in this verse has reign. Root meaning of the word reign is kingdom, to hold or exercise sovereign power. This is an enemy entity. The entity’s job is to get you to do its bidding.

If it can get you to obey it then you will wear a mask because you are not who Christ has made you and you will walk out the flesh with all its deeds instead of walking in the spirit and its fruits. This entities job is to get you to think and believe that the commands are coming from you : THE SINNER!

Romans 6:6 For we know that our old self was crucified with him so that the body ruled by sin might be done away with, that we should no longer be slaves to sin—

If you have received Christ then your old self is dead! The old self was the one who was sold into slavery to sin through Adam.

Romans 5:12[ Death in Adam, Life in Christ ] Therefore, just as sin came into the world through one man, and death through sin, and so death spread to all men because all sinned—

Romans 6:14 For sin will have no dominion over you, since you are not under law but under grace.

  That self that is THE SINNER who SINS under SIN as a SLAVE is NO MORE!

In reality YOU are in CHRIST you are now ALIVE in Christ. You are no longer UNDER SIN!

Romans 7:20 Now if I do what I do not want, it is no longer I who do it, but sin that dwells within me.

Its your choice to walk in FAITH -you are free or you will yield to an enemy entity trying to gain control through your God given ability to believe.

For a great teaching on this from John Piper!!

http://www.desiringgod.org/resource-library/sermons/do-not-let-sin-reign-in-your-mortal-body-part-1

http://www.desiringgod.org/resource-library/sermons/do-not-let-sin-reign-in-your-mortal-body-part-2