The subject of Romans 6 is about being united with Christ, including the experience of death and resurrection with Christ (Romans 6:1-5).  Paul specifically mentioned that our old man was crucified with Christ.  The sinful flesh has already been annulled (Romans 6:6-16), so we should consider ourselves alive to God in Christ Jesus (Romans 6:11).   Although many Christians know this truth, they often find that their old man and flesh are still very active in their experience. How do we make sense of this? Paul's explanation in verses 12-23 says that we should offer our bodies to God as instruments for righteousness and not offer them to sin as instruments for unrighteousness (Romans 6:13).  In other words, as Paul said in Romans 8, "Setting our mind on the Spirit is life and peace, but setting the mind on the flesh is death." (ESV, Romans 8:6).