The founders wrote, and the state's ratified Article VI, Section 3, of the U.S. Constitution which states, "...no religious test shall ever be required as a qualification to any office or public trust under the United States."
Additionally, the First Amendment, added to the Constitution as part of the deal to get it ratified, says, “Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the government for a redress of grievances.”
The Library of Congress published a piece entitled "The Founding Fathers and Islam” in 2003 and that hallowed institution concluded, “The Founders of this nation explicitly included Islam in their vision of the future of the republic.”
However, in all honesty, whether our nation was founded as a Christian or not is an argument best left to academics debating over the dusty tomes of history.
America was definitely founded as a nation that counted African Americans as three-fifths of a human being (U.S. Constitution, Article I, Section 2, but the language was wisely scrubbed by the Fourteenth Amendment) and denied women the right to vote (took them until 1920 until someone wised up and enfranchised the all the ladies, credit to Wyoming for giving them the vote first).
We evolved for the better. The slaves secured their natural right to liberty. Women secured their natural right to vote.
Today, America is a multi-ethnic and multi-faith society with a shared set of core values: individual freedom and dignity, rule of law under which everyone is treated equally, individual opportunity, all committed to fostering the common good, etc.
Christianity is there, contributing to the good of our nation. But so are Judaism, Hinduism, Islam, Buddhism and a host of other faiths.