"Our Constitution was made only for a moral and religious people. It is wholly inadequate to the government of any other." John Adams, Letter from John Adams to Massachusetts Militia, 11 October 1798.
What was it that John Adams meant by this? Why was it so important to him that people be moral and religious? Why cannot the laws of the land assure that regardless of the people, that if the law was enforced, it would keep them within certain lanes of conduct regardless of their individual ideas on morality and religion?
When morals or principles no longer have a religious basis - when they are no longer part of a sacred order - they run the risk of being little more than opinions.
For a nation where consent of the governed is fundamental, and avenues exist by which the rules could change, moral principles would have to be deeply imbedded in the fabric of each individual. Our Founding Fathers were very much constructing a government for morally advanced people. Their principles of limited government required that the people themselves increase their sense of virtue in order for government to step back.
"only a virtuous people are capable of freedom. As nations become corrupt and vicious, they have more need of masters." - Benjamin Franklin
And that's not to say that people without clearly articulated moral principles or without religious guidelines cannot act morally. Many in those categories do and sometimes to a greater extent than those who claim to abide by those guidelines. But once those morals are based solely on the ideas of other human beings, they run the risk of becoming little more than opinions which can change based on the aesthetic views of each generation, leading to instability.
The sentiment John Adams expressed was held not just by him alone, but by the people as a whole at the time of the founding. Massachusetts in 1789, Georgia in 1785, and North Carolina in 1789 all had charters for education that put this principle into law.
Here’s Georgia's, and the others are very similar:
"As it is the distinguishing happiness of free governments that civil order should be the result of choice and not necessity, and the common wishes of the people become the laws of the land, their public prosperity and even existence very much depends upon suitably forming the minds and morals of their citizens. When the minds of people in general are viciously disposed and unprincipled, and their conduct disorderly, a free government will be attended with greater confusions and with evils more horrid than the wild, uncultivated state of nature."
The charters for these state universities focused on the happiness of the people being connected to virtue, and assumed the states should be involved in inculcating that virtue amongst the people.
Concepts like the ones stated in the charters for education were not relegated to educational documents. They made their way into the justification for education in legal documents as well.
"Religion, morality, and knowledge, being necessary to good government and the happiness of mankind, schools and the means of education shall forever be encouraged." - Northwest Ordinance first passed by Congress in 1787 and then again after the Constitution went into affect in 1789
It sounds like the Founders believed education should at least in part focus on religion. In a nation that has removed any aspect of religion from its public schools as a badge of progress, this seems counterintuitive.
One can debate the legality of including religious ideas in schools today, but there is no doubt that when this nation was formed, education was thought of as necessarily including moral principles based on religious ideas.