The earliest colonists -- both the Pilgrims and the Puritans -- were very definitely Christians so the foundation of America was Christian. Later, when America was established as a nation, the Founding Fathers designed the Constitution on Christians principles of liberty and justice for all. While many of them were Deists and Freemasons, others were definitely committed Christians.
George Washington was clearly a Christian:
"Throughout his time as a leader, in the army and civilian government, Washington encouraged the declaration of days of prayer and fasting as a means of seeking divine assistance in perilous situations and also supported the proclamation of days of Thanksgiving to praise God for his help."
https://www.mountvernon.org/george-washington/religion