A good deal of the new testament seems to be anti-circumcision (for instance, Paul says "if you let yourselves be circumcised, Christ will be of no value to you at all" in Galatians 5:2). Yet today, in America at least, most Christians circumcise their children. When and why did this shift in attitude towards circumcision occur?
In the context of the complete passage, Paul's statement there in Galatians is not a condemnation of circumcision per se, but rather a condemnation of those who merely follow Jewish law and customs without faith. The next few lines (Galatians 5:3-6, NRSV) make this clear:
Once again I testify to every man who lets himself be circumcised that he is obliged to obey the entire law. You who want to be justified by the law have cut yourselves off from Christ; you have fallen away from grace. For through the Spirit, by faith, we eagerly wait for the hope of righteousness. For in Christ Jesus neither circumcision nor uncircumcision counts for anything; the only thing that counts is faith working through love.
So there is not an explicit ban on the physical act of circumcision itself, only a proclamation that it and other traditional Jewish rites are meaningless to Jesus. There is nothing either prohibiting or requiring the foreskin be removed; Paul uses "circumcision" as a broad term for the Jewish law to illustrate his point. As he does elsewhere throughout the New Testament, notably in Romans 2, where he even goes so far as to say "Circumcision indeed is of value if you obey the law" and also explicitly states that "Real circumcision is a matter of the heart—it is spiritual and not literal."
It should also be noted that Paul had his follower Timothy circumcised and that Jesus was himself of course circumcised.
So, long story short, there is no direct condemnation of the physical act of circumcision itself in the New Testament, only of those who would use it and other Jewish rites as external replacements for internal faith.
That being said, it is not exactly true that circumcision has become widespread throughout the Christian world. It is true that the practice has become more common in the United States than Europe, and that the United States is also now generally more religious than Europe, but as others have stated that is due more to cultural than religious factors. Since the New Testament does not in fact explicitly forbid or mandate the act, religious factors would be largely irrelevant to the decision for Christians, unlike Jews and Muslims.