Christianity, part of the Judeo-Christian religions along with Judaism and Islam, is the belief in Jesus Christ as the Messiah. Followers of Christianity are called Christians, and their holy book is the Bible. Christianity is an incredibly broad religion with many variants from conservative to liberal, and from literal to highly figurative, but the vast majority are based on their interpretation of the teachings of Jesus Christ (whether he is regarded as real, or less commonly, metaphorical).
Regarding Body Modification
While some people who identify as Christians may have their own personal or political objections, there is nothing explicit in the Bible to suggest that piercings and tattoos and even extreme body modification is "un-Christian".
The Bible is full of pierced and tattooed individuals, both good and bad, and history contains many examples of tattooed Christians. For example, the crusaders were often heavily tattooed with Christian iconography specifically as part of their faith, and some modern Christians use piercing to represent bondage to God.
People ignorant of its actual meaning often quote Leviticus 19:28, which reads, "Ye shall not make any cuttings in your flesh for the dead, nor print any marks upon you: I am the LORD." Like much of Leviticus, this law was intended to differentiate the Jews from the pagans. The actual word used ("k'thoveth qa'aqa") is a reference to a form of cutting with ash rubbing, a pagan funerary rite at the time. So this law isn't there to stop people from having tattoos, it's to keep people from performing the rites of other religions.
In any case, the new covenant decrees that modern man is justified through acts of faith, not act of law (Romans 3:28, a reference to circumcision being unimportant). Christians are no longer banned from cutting their beards and all the other old laws, so long as they remain good Christians (not that there's much agreement on that).
- Does God Hate Your Tattoos? (BME/News).