“For in Him all the fullness of Deity dwells in bodily form, and in Him you have been made complete, and He is the head over all rule and authority; and in Him you were also circumcised with a circumcision made without hands, in the removal of the body of the flesh by the circumcision of Christ; having been buried with Him in baptism, in which you were also raised up with Him through faith in the working of God, who raised Him from the dead. When you were dead in your transgressions and the uncircumcision of your flesh, He made you alive together with Him, having forgiven us all our transgressions, having canceled out the certificate of debt consisting of decrees against us, which was hostile to us; and He has taken it out of the way, having nailed it to the cross. When He had disarmed the rulers and authorities, He made a public display of them, having triumphed over them through Him.
Therefore no one is to act as your judge in regard to food or drink or in respect to a festival or a new moon or a Sabbath day - things which are a mere shadow of what is to come; but the substance belongs to Christ.” Colossians 2:9-17
The reason a Christian can think like a new man and, therefore, act like a new man is because he is a new man in Christ Jesus. We tend to view the Christian life like a patched tire. We think of salvation as something added to our lives when we accepted Christ, repairing the puncture of sin but still leaving us damaged. That is not the biblical version at all. When we accepted Christ, we were given a new nature, Christ’s nature. We were give a brand new tire so to speak, not a retread.
You are no longer in Adam but in Christ. You are not partly in Christ and partly in Adam but completely in Christ (Col. 2:9). You have exchanged your old corrupt sin nature for Christ’s holy nature. This happened at the cross. You were crucified with Christ, buried with Christ, and risen with Christ. God did it, placing you in Christ and Christ in you (1 Cor. 1:30).
The life of Christ is your life. You can conquer any sin habit because you are now a slave of righteousness, not a slave of sin. As a new creature in Christ, you are free to serve Christ. You still may choose to sin, but you don’t have to for you have died to sin and are fully alive to God in Christ (Rom. 6:6-11).
“...our old self was crucified with Him, in order that...we would no longer be slaves to sin.” Romans 6:6