Colossians 2:11

"In him you were also circumcised, in the putting off of the sinful nature, not with a circumcision done by the hands of men but with the circumcision done by Christ," NIV translation

Circumcision of the flesh is the process of cutting off excess skin that is not necessary and indeed can be harmful to cleanliness. The process was done by a priest as prescribed by God in the Old Testament. Paul brings this up to address the third challenge that had got into the church which was legalism. Here, he reminds the believers that our circumcision is not of man's rules but by the very Spirit of God. 

We all go through a process of cutting out the flesh from our lives when we come to Christ. The old self (flesh) is removed just as with the Israelites but it is accomplished through the Holy Spirit and not man or man's laws.

Colossians 2:12

"having been buried with him in baptism and raised with him through your faith in the power of God, who raised him from the dead." NIV translation

Our circumcision is accomplished by faith as our old self dies and is buried (baptized) and we are raised again just as Jesus was to a new life.

Colossians 2:13a

"When you were dead in your sins and in the uncircumcision of your sinful nature, God made you alive with Christ." NIV translation

We all at some point were dead men walking as we went about our lives in sin. We followed the ways of the flesh (sinful nature) and were headed in the direction of death because that is the penalty for sin. But, when we were powerless to help ourselves, God sent His Son to bring us new life.