Keep your eyes posted, I’m going to post something about Machen and Catholics later this week. The broader context of what he said may surprise you.
If it's the item I think you're referring to from “Christianity and Liberalism” in which Machen advocated what (for his day) was a shockingly positive evaluation of the Roman Catholic Church, it's important to remember that Machen understood the strategic alliance that Abraham Kuyper had made with Roman Catholics two generations earlier in the Netherlands — an alliance that made the Netherlands into the “Bible Belt” of Europe for well over three-quarters of a century, and was virtually unique for Europe in turning back the progress of liberalism in the 1800s and early 1900s.
Machen and Kuyper both understood that Roman Catholics and evangelical Protestants, despite very serious differences, had far more in common than either had with liberalism. Kuyper succeeded in convincing Roman Catholics that their historic alliance with liberals was contrary to their own confession, much as the American religious right in the 1960s, 1970s and 1980s convinced conservative Roman Catholics that the issue of abortion warranted breaking their historic ties with the Democratic Party.
Machen believed — correctly so — that liberalism not only is not Christianity but that it belongs in an entirely different category of religions from Christianity.
Christianity is a revealed religion based on submission to external authority.
Liberalism is a man-centered religion which takes a variety of different forms, most commonly some combination of deification of personal mystical experience (i.e., Schleiermacher) and deification of unaided human reason (i.e., Von Harnack).
When a liberal accuses a conservative Protestant of making the Bible into a “paper Pope,” that liberal has actually identified a real similarity between traditional Roman Catholics and evangelical Protestants — submission to external revealed authority.
But by making that accusation against conservative Protestants, the liberal is not really attacking Protestants for being hypocrites but rather showing just how far away from revealed religion liberalism actually is.