Romans 6:12

Let not sin therefore - This is a conclusion drawn from the previous train of reasoning. The result of all these considerations is, that sin should not be suffered to reign in us.

Reign - Have dominion; obtain the ascendency, or rule.

In your mortal body - In you. The apostle uses the word “mortal” here, perhaps, for these reasons,

1. To remind them of the tendency of the flesh to sin and corruption, as equivalent to “fleshly,” since the flesh is often used to denote evil passions and desires (compare Romans 7:5, Romans 7:23; Romans 8:3, Romans 8:6); and,
2. To remind them of their weakness, as the body was mortal, was soon to decay, and was therefore liable to be overcome by temptation. Perhaps, also, he had his eye on the folly of suffering the “mortal body” to overcome the immortal mind, and to bring it into subjection to sin and corruption.

That ye should obey it - That sin should get such an ascendency as to rule entirely over you, and make you the slave.

In the lusts thereof - In its desires, or propensities.