1 Corinthians 7:18
Is any man called being circumcised? let him not become uncircumcised. Is any called in uncircumcision? let him not be circumcised.
Note 3 at 1Co 7:18: What does it mean to become uncircumcised? How could that be? It is a fact that in Paul's day, the physical appearance of circumcision could be reversed (Adam Clarke's Commentary), similar to the way modern surgery could change the appearance today. But that is extreme and not what Paul was talking about here.
The word "circumcision" referred to more than the physical act. It was a condition of the heart (see note 4 at Ro 2:29), and in a broader sense, it referred to the fact that an individual was a Jew (see note 1 at Ac 11:2). Therefore, Paul was simply stating that those who were Jews before they were born again should not renounce their Jewish heritage. Likewise, Gentiles who come to Christ do not have to become Jewish.
Failure to understand this truth has caused some Christians to feel they have to embrace the Jewish practices prescribed in the Old Testament. Others, who were Jews, are sometimes led to believe they have to sever all links with their Jewish heritage. Neither is true. The only thing that matters to God is our new identity in Christ (Ga 6:15).