What is the believer admonished to do concerning the old Adam?
The believer is admonished to reckon himself dead indeed unto the old Adam and alive unto God through Jesus Christ, and to let not old Adam continue to reign in the mortal body. This requires choosing to be led of the Spirit and keeping old Adam in subjection.
Les explains that “reckon” means to come to a mental conclusion, and the believer now has freedom and must decide whether to live a spirit-filled life or a carnal, fleshly life. The admonition is to be led of the Spirit, to live profitable lives, and to bring honor and glory to God by keeping old Adam in subjection. Les also points out that “let not” implies choice: it is not forced on the believer, but one chooses not to let old Adam continue to reign in the mortal body. Many unhappy believers are miserable because old Adam is still controlling them, though he does not have to.