He exposes the problem of revisionist history, which tries to establish what it thinks should have happened rather than what did.
The student of history need not be a Christian and is free to be overtly hostile to Christianity, but primary sources should not be edited in order to create a de-Christianized myth of American history.
As Alexis De Tocqueville says in the quoted section of his work,
"I do not know whether all the Americans have a sincere faith in their religion, for who can search the human heart? but I am certain that they hold it to be indispensable to the maintenance of republican institutions. This opinion is not peculiar to a class of citizens or to a party, but it belongs to the whole nation, and to every rank of society."
Even free thinkers like Thomas Paine, who was critical of the Christian religion, could speak the language of the Bible and draw positive moral arguments from it. Biblical allusions and phrases filled public discourse until only a few decades ago.
It is not so today; even Christian researchers like George Barna find Biblical and theological illiteracy in the churches themselves. But that only points to changes in progress, not a redefinition of what was - and what was formative and worth recalling as a healthy challenge to new truth claims and newly minted myths.