Colossians 2:11 says, “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;”
In Christian salvation the old man, the material part of your body, is not converted or rehabbed but it is removed. It’s important to note that Christianity is not intended to be in anyway shape or form the renewal of yourself. Christianity is intended to make you a new creature.
Paul says this in 2 Corinthians 5:17: 17 Therefore if anyone is in Christ, he is a new creature; the old things passed away; behold, new things have come.
There is never an emphasis on a Christian disciplining himself to become moral. There is the emphasis on the Christian being born again as a new creature with a new nature whose being is no longer dead towards God but alive to Him and in a living relationship with Him.
Verses 12 & 13 describe this: 12 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. 13 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,”
Verse 11 in Colossians 2 is not allegorical or figurative but fact. In Christian salvation your inner man or soul is regenerated from a soul whose nature is sin to one whose nature is divine. This is what must happen in salvation for Jesus talked often about sin coming from within a man. It’s the inner man that needs converting not the old man. The old man shows or manifest what is within a man.
In other words the old man is not the problem or the source of the problem it’s the inner man. This is why the old man is circumcised or removed spiritually. Because it has been corrupted from the sin flowing from the original soul in a man and therefore it cannot inherit the Kingdom of Heaven.
Interestingly in Christian salvation the corrupted old man does remain with us when we are born again which creates the battle between the old and new man or the flesh and Spirit as Paul writes. Therefore in practice the old man is mortified by the Holy Spirit [see Romans 8:13) which is a demonstration of it being circumcised as Colossians 2:11 says.
But the point today is the goal of Christianity is not a reformation of the old man but a rebirth of the inner man from which flows the Spirit of God and therefore brings forth divine works or fruits of the Spirit.