The majority of them were.
John Adams was one of the more vocal:
“The general principles upon which the Fathers achieved independence were the general principles of Christianity…I will avow that I believed and now believe that those general principles of Christianity are as eternal and immutable as the existence and the attributes of God.”
[June 28, 1813; Letter to Thomas Jefferson]
"We have no government armed with power capable of contending with human passions unbridled by morality and religion. Avarice, ambition, revenge, or gallantry, would break the strongest cords of our Constitution as a whale goes through a net. Our Constitution was made only for a moral and religious people. It is wholly inadequate to the government of any other." --October 11, 1798
Thomas Jefferson and some of the other founders were deists. They believed a higher power set the universe in motion, then left it alone. The term "separation of church and state" is from a letter Jefferson wrote to the Danbury Baptist Association:
"I contemplate with sovereign reverence that act of the whole American people which declared that their legislature should 'make no law respecting an establishment of religion, prohibiting the free exercise thereof,' thus building a wall of separation between church and State."
When Thomas Jefferson wrote his letter to the Danbury Baptist Association he never intended the words "Separation of Church and State" to be taken out of context and used as a substitute for the First Amendment, but for all practical purposes that is what the courts have done. The wall was between practitioners of peaceful faiths and the federal government's intrusion in any capacity, not between the peaceful practitioners and their ability to practice how they want where they want.
What the 1st amendment protects against is the government establishing a state religion AND infringing on any citizens' right to practice their religion. It does not permit the federal government to stop someone from practicing their religion anywhere unless they are violating the life, liberty or private property of another individual. The terminology "Congress shall make no law..." could not be clearer. The restriction is on the government, not the people.
The nation was founded on the principles of Natural Law which are generally in congruence with Christian ideals. Humanity has 3 inalienable rights - Life, Liberty, and Private Property. Humans created governments to protect those rights. Because governments write laws and law's sanction is force, government's only role can be to protect rights. Because governments are made up of men with the same fallibility as everyone else, they are no better able to direct individuals than free individuals direct themselves. Therefore, to keep men free government must be kept strictly limited.
Washington rarely discussed or wrote about his religious and philosophical opinions in any great detail yet he frequently displayed a humble and gracious respect towards God in his personal letters and public speeches. Washington regularly attended Sunday services and purchased a family pew at several churches. Rev. Lee Massey, his pastor in Mt Vernon, wrote "I never knew so constant an attendant in church as Washington."