To reign is to have dominion. We are free from bondage and ought not to allow ourselves to come back under it.<p>Note that it says do not let sin <i><b>reign</b></i> in your mortal body. It cannot reign in your spirit which is perfect. Your body has not yet been redeemed. It is corruptible. If will one day put on incorruption. At that time, it will be immune from sin.<p>But, at the moment, your body is mortal meaning it is subject to death. So, you must mortify the lusts of the flesh in order to preserve it. - Col 3:5