Here is Rob Boston from American's United defending the Everson case, which has become a bugaboo of the religious right. Boston takes on both the clownish religious right figures like David Barton but also the more serious legal scholars who criticize Everson like Philip Hamburger and Daniel Dreisbach.
Nevertheless, several right-wing scholars have accused Black of being anti-Catholic, among them Dreisbach, Philip Hamburger and even Jay Sekulow, TV preacher Pat Robertson's top lawyer.
In his 2006 book Witnessing Their Faith: Religious Influence on Supreme Court Justices and Their Opinions, Sekulow notes that Black grew disillusioned with the Baptist faith he was raised in and in Washington attended a Unitarian church. Sekulow hastens to add, "While many of the theological doctrines and practices of the Baptist denomination did not appeal to Black, their separationist and anti-Catholic declarations found a deep resonance within him."
Sekulow's source for this is the writings of Hamburger, who accuses Black of anti-Catholicism as a way to impugn the separation concept. Hamburger's magnum opus is the misnamed Separation of Church and State, a 492-page screed against that principle. The reasoning is somewhat circular: Black was an anti-Catholic bigot. Therefore, Black supported the separation of church and state. Therefore, support for separation of church and state means you are an anti-Catholic bigot.
Yet the question of Black's alleged anti-Catholicism is not so simple. Some Black biographers, primarily Steve Suitts in his book Hugo Black of Alabama, defend Black against the charge.
I've reviewed the first half of Hamburger's book here where I analyze how he treats the notion of "separation of church and state" in the Founding era. I don't address the second half of his book where Hamburger deals with Everson. Hamburger is a brilliant scholar who has done impressive research on the matter; so I take his theories with a high degree of charity and deference. I will say that the weakest part of his book is his analysis -- how he puts the facts, which he meticulously researches, together. Arguably the notion that Hugo Black gave us Everson because he was a Klan imbibed anti-Catholic bigot is a smear against Black.
Still, there is a serious scholarly case to be made that Everson errs simply because the Establishment Clause is not incorporable, and that much of what Hamburger puts forth regarding whether 18th Century dissenters believed in "separation" or whether Black et al. were anti-Catholic bigots, is entirely besides the point. For a work that makes the argument against the Establishment Clause's incorporation without stepping into these extraneous matters, see Vincent Phillip Munoz's The Original Meaning of the Establishment Clause and the Impossibility of Its Incorporation, 8 University of Pennsylvania Journal of Constitutional Law 585-639 (2006).