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 (KJV)

Then: Paul never leaves his hearers without examples. The original question could have come from the Corinthians, or it could have come from other churches. Along with having both a physical and spiritual meaning, the query fits into his current teaching point and principle of "Stay where you are when called by the Lord." Notice that "circumcision" is another way for Paul to say "Jewish," and "uncircumcised" is a way to refer to "Gentiles" or "pagans." There is no reason to become circumcised when the people the Corinthians would influence about Christ's lordship are those who are already associates. Most likely they were not circumcised. Vice-versa, if a Corinthian was a Jew, the people whom they would influence most are other Jews. The spiritual meaning is the Corinthians' daily task of telling others about Christ.

Now: Today's Christians use the same principle, inasmuch as it is possible. If they are able to speak with past friends individually without being pulled back into a former life - such as the drug culture or gang mentality, then they are free to keep close associations. However, if the influence of the other is greater than the believer's strength of purpose, close associations are not advisable. Anyone looking at Paul's biography notices the multi-year gap between his conversion and his ministry. Before turning to a life of daily service to the Lord Paul took time to withdraw and re-orient his world view by studying Christ. Believers need both Paul's teaching and example in order to be effective followers.