Circumcision is the covenant between God and Abraham, Genesis 14. This is later reemphasized in Exodus, when God loses his shit over Moses' uncircumcised sons.
However, Jesus establishes a new covenant, which is no longer exclusive to Jews, as Abraham's was. This leads to the argument made by Paul, that the new covenant means there is no longer a circumcision requirement for Christians (Galatians 5 & 6, Romans 2 & 3, Corinthians 7, Colossians 3, list goes on). This goes so far that Paul eventually comes to use "People of circumcision" as an euphemism for "Jews" (Titus 1), and it's not in an appreciative context, to say the least.
Paul has a good case, too. The new covenant replaces the old one, and Matthew 28 is explicit about the new covenant no longer being exclusive to Jews.
Basically, the bible tells us that
- circumcision equals Jewish
- circumcision is not a requirement for Christians
So after almost 2000 years of Christians rejecting circumcision both in their holy book and in practice, various flavors of American protestants in late 19th century to current America decided that circumcision is a splendid idea.
Make of that what you will.