Colossians 2:11 (and following) does not teach that the Christian's salvation results automatically in the inability to sin. The old nature, as Paul defines it, was dealt a death blow at the cross of Christ, but we as Christ followers need to put it to death regularly. That is why Jesus told his followers to take up their cross daily and follow him.
In Colossians, as elsewhere in Paul's writings, Paul distinguishes between our standing in Christ, on the one hand, which is once and for all, and our state in Christ, on the other hand, which varies from day to day and sometimes even from minute to minute! Sinless perfection is not possible, and Paul does not suggest it is.
A proper understanding of the phrase "the circumcision of Christ" is crucial in this regard. Verse 12 tells us clearly that Christ's circumcision is a kind of death. At conversion, when Christians embrace Christ's death on their behalf for the forgiveness of sins, metaphorically they too die. Paul calls it being "buried with Him in baptism."
Burial, of course, signifies separation of the spirit from the body through death. That separation for Christ happened literally at the cross. When he had made full atonement for the sins of the world and said "It is finished," he then committed his spirit into the hands of his Father and died (Luke 23:46).
For his followers, however, that separation--though not the separation of spirit from body--happens figuratively. The figure Paul uses here is that of circumcision. It is, as Paul puts it,
". . . a circumcision made without hands, in the removal of the body of the flesh by the circumcision of Christ" (v.11).
As for our spiritual standing before a holy God, we in Christ have been made complete (v.10). In 2 Corinthians chapter 5, Paul expresses this thought differently by saying that we in Christ "have become the righteousness of God" (v.21). We cannot get any better than that, positionally.
As for our spiritual state before a holy God, our challenge as Christians is to "reckon" ourselves to be dead unto sin.
. . . reckon ye also yourselves to be dead indeed unto sin, but alive unto God through Jesus Christ our Lord (Romans 6:11).
This reckoning ourselves (or considering ourselves) dead to sin is perhaps the most challenging aspect of discipleship. Had God, upon our conversion, made sinning impossible, then how could our obedience to Christ ever be a conscious act of love? It would not.
After conversion, as before conversion, God gives us the ability to make decisions and to choose good or evil. The battle of "the body of flesh" versus the regenerated spirit is a life-long struggle, as Paul found out the hard way (see Romans 7:14 ff.). Without this lifelong struggle, we could not possibly grow in Christ Jesus, nor would our sanctified lives bring glory to God if there was not even the potential to sin.
Sinless perfection, therefore, is but a pipe dream and is unachievable this side of heaven. Progressive and lifelong sanctification, that gradual process of growing in Christ and bearing fruit which remains (see John 15) is made possible through a daily, intimate relationship with God whereby we decide to "let go and let God" (as those who are struggling to recover from addiction like to say). This letting-go process is the only way God is then free to do in our lives what he needs to do.
Yes, we are to "work out our own salvation," but always in concert and in cooperation with
God, realizing that
. . . God is at work in us to will and to do his good pleasure (Philippians 2:13).
In conclusion, the separation of our souls from sin involves a figurative circumcision. Just as a literal circumcision removes a part of a man's anatomy from him while the rest of the body lives, so also does the figurative circumcision made "without hands" underscore the importance of living separated lives, separated from the body of flesh (or simply "the flesh"--in Greek, sarx), but alive unto God through daily crucifixion and resurrection to newness of life.