Circumcision was a big deal to Jewish people during the writing of the New Testament. It became the sign that you were “in.” Paul battled against this external act as no proof of salvation. It was not an easy battle and it’s one that still rages.
There are two kinds of legalists in the world:
Those who think they must do external religious rites to be righteous
Those who think you better not do external religious rites to be righteous.
It’s amazing how human nature can turn liberty into a law! Liberty says I don’t have to do stuff so therefore you are wrong for doing that stuff. Nope, liberty says it doesn’t matter one way or the other!
External religious rites, following the commandments of men, or any other act does not determine your righteousness before God. Here is what matters, it mattered in the area of circumcision and in every other area:
“For in Christ Jesus neither circumcision availeth any thing, nor uncircumcision, but a new creature.”
Regeneration is the issue! You must be born again. You can’t rebirth yourself. You had no say in your physical birth and you shouldn’t imagine you can create your spiritual birth. You cannot impress God apart from the faith that leads to regeneration.