Civil religion, as Pierard (a Baptist university professor writing in 1988) tells it, is religious statements, claims, and exhortation that's publicly displayed in society (especially by public officials) with the assumption that everyone or almost everyone can join in it.
Pierard talks some about religion in America at the Founding, and then goes through several Presidents to search out their personal religious beliefs and how they participated in civil religion. He makes interesting points about how America's civil religion has shifted and become more broadly inclusive, and about how Presidents can engage in civil religion as either a prophet or a priest. He favors the prophetic role, and I see why - but that assumes the prophet is speaking the truth! Have not both Biden and Trump acted as diametrically-opposed "prophets" on race relations? Does not most of America hate at least one of them as a prophet?
His telling of different Presidents' religious lives is also interesting. All of them called themselves Christians - yet I find myself doubting the real Christianity of some (like Wilson) and firmly resolved against it for others (like Eisenhower).
That said, the book goes on for too long with too much density for me to really recommend it.