In the 19th century, with the wave of Irish immigrants, there were public schools in big cities which taught “Religion,” which meant generic Protestantism, and how could that possibly be a constitutional issue? It is Americanism itself. It’s not like the schools were pushing a particular form of Protestantism, so no one’s religious liberty was implicated, no religion was being established. And anyway Catholics are reactionary pawns of a foreign Prince, being damned Papists, and thus presumptive traitors. This was the view. It was a huge issue, nicely described in Philip Hamburger’s Separation of Church and State. Catholicism is not a real religion (a view that still survives in some quarters), Catholics are not real Americans, probably dangerous strangers in our midst, etc. etc. It was for this reason that Catholics tried to get public funding for their schools in the 19th century (after all, they pay taxes, so why should their children be whipped for refusing to read the King James Bible in public schoolrooms?), and Protestants tried to stop them, which is the 19th century forerunner to the school choice movement and resistance to it. And no, Irish-American Catholics and Italian-American Catholics were not altogether wild about US involvement in wars on behalf of the Ireland-oppressing UK, or against Italy either.
Happily, we now see the error of our ways, and know who the real strangers in our midst are.
Insert Santayana quote here.