The United States of America was never intended to be a Christian nation, and as President Obama just recently declared, "...we do not consider ourselves a Christian nation or a Jewish nation or a Muslim nation." The First Amendment of our Constitution clearly defines this, and as a country, we should be able to recognize the importance of the separation of church and state. This phrase was not present until the mid-1950s, when the US was going through the Red Scare (a period where Sen. Joseph McCarthy called out many public figures on being "Communists", subsequently ruining their reputations). It does not belong on our money, and it should be removed as soon as possible.
In 1947 (Everson v. Board of Education), the Supreme Court of the United States ruled the following concerning the Establishment Clause (First Amendment, US Constitution):
"The 'establishment of religion' clause of the First Amendment means at least this: Neither a state nor the Federal Government can set up a church. Neither can pass laws which aid one religion, aid all religions, or prefer one religion over another. Neither can force nor influence a person to go to or to remain away from church against his will or force him to profess a belief or disbelief in any religion."
"The government of the United States of America is not in any sense founded on the Christian religion." - Treaty of Tripoli (1797)