Reddit reviews Metaphysics: The Fundamentals (Fundamentals of Philosophy)

I think you will learn the most by reading five textbooks, such as A History of Philosophy, volumes 1, 2, 3, 4, and 5; or something like Metaphysics: The Fundamentals, The Fundamentals of Ethics, Theory and Reality: An Introduction to the Philosophy of Science, An Introduction to the Philosophy of Religion, and An Introduction to Political Philosophy.

If what you have in mind is more of a "Great Books" program to get your feet wet with some classic works that are not too difficult, you could do a lot worse than:

• Plato's Euthyphro, Apology, Crito, and Phaedo, often published together under the title The Trial and Death of Socrates. Socrates is so important that we lump together all Greek philosophers before him as "the Presocratics," and this cycle of dialogues is a great window on who he was and what he is famous for.
  
• The Basic Works of Aristotle. "The philosopher of common sense" is not a particularly easy read. Cicero compared his writing style to "a flowing river of gold," but all the works he prepared for publication are gone, and what we have is an unauthorised collection of lecture notes written in a terse, cramped style that admits of multiple interpretations. Even so, one can find in Aristotle a very attractive system of metaphysics and ethics which played a major role in the history of philosophy, and holds up well even today.
  
• René Descartes, Discourse on the Method and Meditations on First Philosophy. Descartes is called the father of modern philosophy, not so much because modern philosophers have widely followed his particular positions (they haven't) but because he set the agenda, in a way, with his introduction of methodological scepticism.
  
• David Hume, An Enquiry Concerning Human Understanding. I think Elizabeth Anscombe had it right in judging Hume a "mere brilliant sophist", in that his arguments are ultimately flawed, but there is great insight to be derived from teasing out why they are wrong.
  
• If I can cheat just a little more, I will lump together three short, important treatises on ethics: Immanuel Kant's Grounding for the Metaphysics of Morals, John Stuart Mill's Utilitarianism, and Anscombe's paper "Modern Moral Philosophy".

>One could claim that you were also taught to parrot that particular line as part of being Catholic. But that alone isn't grounds for validating/invalidating something, surely?

It's absolutely a justified reason for dismissing an argument if no reasoning is given.

>Also isn't "metaphysical grounding" an oxymoron? Metaphysics is anything but grounded.

I don't think you know what metaphysics is.

Start here: https://plato.stanford.edu/entries/metaphysics/

Good textbook on contemporary analytic metaphysics: https://www.amazon.com/Metaphysics-Fundamentals-Philosophy-Robert-Koons/dp/1405195738