N this chapter, Paul ably addresses the relationship of the Jews (the circumcised) with the Gentiles (the uncircumcised). It is rather clear that Paul did not consider the differences between the two to be in the least bit significant – not if the economy of heaven is the measuring standard. Paul even intentionally refused to allow one of his apprentices (Titus) to be circumcised (because it was important that Paul contribute continually to an untainted definition of the gospel). This was a bold move indeed, especially considering that Paul took Titus with him when he went to Jerusalem to report to the church leaders there concerning his successful ministry of the gospel among the Gentiles. While we know very well how willing Paul was to adjust & to make concessions in order to spread the gospel, he was also willing to be infinitely stubborn in defense of that same pure gospel (vs. 4-5).
In summation, Paul declared that he was (& we – both Jews & Gentiles – are) "crucified with Christ'' (vs. 20). On top of that, now Christ lives in us. We died in Him & now He lives in us. This is the reason & the reality of our salvation. Safe from hell because the debt & penalty of my sin has been paid in full. There is no double jeopardy in God's plan of redemption. When He was on the Cross, I was there with Him. It is the height of the vicarious ministry of Christ on our behalf. He became sin for us so that we could become righteousness (2nd Corinthians 5:21). We are justified (declared to be innocent) due to His finished work on the cross, & our faith in it.