When you came to Christ, you were “circumcised,” but not by a physical procedure. Christ performed a spiritual circumcision—the cutting away of your sinful nature. 12For you were buried with Christ when you were baptized. And with him you were raised to new life because you trusted the mighty power of God, who raised Christ from the dead.
We can see in Verse 11 that Paul was referring to the Jewish tradition that was being forced upon the gentiles - the act of circumcision. Circumcision is no longer seen as a religious thing in our day but i would still share some light it. When we read the old testament and consider the rules therein, we must realise that they were a type of the real thing and not the real thing themselves. Circumcision was instructed by God as a way of showing that Jews were set apart from the world and they are killed the flesh. In salvation, we have the real thing and not just a type.
By getting born again, we are being cut off the life of sin with the deal and brought directly into relationship with Christ and this cutting off is precise and perfect as it is done by Christ.
“by believing in Christ, all was obtained which was required in order to salvation. Circumcision was an ordinance by which it was denoted that all sin was to be cut off or renounced, and that he who was circumcised was to be devoted to God and to a holy life. All this, the apostle says, was obtained by the gospel; and, consequently, they had all that was denoted by the ancient rite of circumcision. What Christians had obtained, moreover, related to the heart; it was not a mere ordinance pertaining to the flesh.” - Albert Barnes NT Commentary
In Christ’s death, we also died to this world and in his resurrection we have been raised to eternal life. We are baptized into Christ’s death and raised into eternal life with him. This is why baptism for us as Christians is like an initiation rite; by being submerged under the water, we represent we death and by being raised again we depict our resurrection into eternal life.