The London Baptist Confession (1689) writes
All things in Scripture are not alike plain in themselves, nor alike clear unto all; yet those things which are necessary to be known, believed and observed for salvation, are so clearly propounded and opened in some place of Scripture or other, that not only the learned, but the unlearned, in a due use of ordinary means, may attain to a sufficient understanding of them. (LBCF 1.7)
The Perspicuity of Scripture asserts: If a doctrine is not reasonably clear to a reasonable reader, then there is no reason to assume that it is important. It may be interesting, and it may be right, but it is not important. As a corollary, you can't prove it to be correct, you can only present plausibility arguments. And failure to affirm such derived doctrines is not heresy or blasphemy, regardless of what any dead white Reformed scholar claims. For if it were a bulwark against blasphemy and heresy, then it would presumably be important and so, by the Perspicuity of Scripture, it would be clear.
The Perspicuity of Scripture and complex, vital, derived doctrines are mutually exclusive. And you can't criticize the Catholics for their derived dogma when you have a corpus of your own.2
1 Disclaimer: I am an R.C. fan-boy. I don't always agree with him, but he makes me think. And like any good teacher, he presents material clearly, concisely, and relatively jargon-free. This is a feature, not a bug.
2 What of the Trinity you say? It is a fair question. I have no good defense other than to say that while the Trinity is a derived doctrine, it is on far more solid ground that many of the esoteric doctrines debated among the uber-reformed.