The decline of Mainline Protestantism, that is the historically prominent American Protestant denominations, had profound consequences for America and American Christianity. Mainline Protestantism was how Christianity was integrated with and represented in society as a whole. With the decline of the mainline denominations, America ceased to be a Protestant, and ultimately Christian nation.
This podcasts describes what Mainline Protestantism is, the origins of the term "mainline", the nature of their decline, their role in the sacred order of society, what may have led to their decline, and the consequences for the church and society.
David A. Hollinger, "After Cloven Tongues of Fire: Ecumenical Protestantism and the Modern American Encounter with Diversity": https://academic.oup.com/jah/article-abstract/98/1/21/873365