Despite many attempts to dispose of religion, human beings have an inbuilt urge to seek out the God who made them (Ecclesiastes 3:11; Acts 17:27).  Even those who scoff at the idea, find it will not easily leave them ... as their atheistic challenges betray.  But for those who want to securely identify themselves with their religion, permanent physical marks, including circumcision and tattoos have been common for centuries.  Of course, it was God who commanded circumcision in the Old Testament as a way of reminding His people about their special identity as His people.   But when Jesus came, as the final revelation of God's identity (Hebrews 1:1-3), everybody who trusts in Him - Jews and many Gentiles (uncircumcised people who were not born into a Jewish family) will be saved irrespective of nationality or religious background.  And so circumcision became redundant. Faith in Jesus and being sealed by the Holy Spirit are what now identify God’s holy people.

The Jews and Jewish-background believers initially demanded that Gentile believers must be circumcised but Paul taught that was not necessary (Galatians 5:2).  He openly condemned Peter for despising uncircumcised Gentile Christians (Galatians 2:11-16).  The birth-mark of the Christian is the gift of the Holy Spirit (Romans 8:9), and His presence is evidenced by the fruit of the Spirit – changed character (Galatians 5:22-26).

Christians do not carry physical marks – our Saviour still bears those (Luke 24:39-40; Revelation 5:6).  Instead we remember the crucifixion wounds of Christ in our temporary encounter with bread and wine (1 Corinthians 11:24-26), and identify with Him in baptism (Romans 6:3-4); but there is no permanent physical mark on our bodies.  We live by faith in the Son of God who loved us and died for us (Galatians 2:20) - the marks were on Him and not on us.  However, the church has always been pressured to think that faith is not enough. Paul repudiates that idea. Indeed he says here that those who follow Jesus have chosen to cut out from their inner desires everything which offends God - not an external mark but a radical internal spiritual surgery (Romans 2:29).  

Whatever pressure you may be under to adopt external religious identifiers, never forget that the Bible does not require any of them.  Even the wearing of a cross or fish badge is not essential.  No, the real mark of believers is that they have the Holy Spirit (2 Corinthians 1:21-22) who enables them to worship and look confidently towards a glorious future when Jesus returns.  We cannot do that by ourselves, however religious we are.  We need the Holy Spirit to work in us, stirring our faith, so that we learn to rely on the promises of God for our salvation and sanctification; and nothing else.  And we need a clear head, lest we get trapped by human inventions which purport to support our faith, but which really deny it.