Where does the new nature in a believer come from?

Les explains that the believer indeed has two natures. We still possess the old nature—the old Adam that constantly pursues us—and we also have the new nature as a result of our salvation experience and our union with Christ. He calls this a Pauline doctrine: the old sin nature is constantly at war against the new divine nature. In the context of dying daily, Les says the old Adam must be kept in subjection and put to death. He points to Romans 6:13 and Romans chapter 6, where Paul deals with reckoning old Adam as our mortal enemy and putting him constantly to death. Although that may sound like double talk, Les explains that God, in His omnipotence and sovereignty, reckoned our old Adam as crucified with Christ the moment we believed. He is dead, yet in experience he is still with us; he plagues us until we leave this body of mortality.