Z << Okay, so why should he be right about the Mormons and wrong about the Catholics? If he was smart enough to figure out correctly that Mormons are not Christians, why should he not be smart enough to figure out correctly that Catholics are not Christians? >>
Oh this is easy.

Matt Slick is mostly right about the Creeds, and but mostly wrong about Catholicism.

When he uses the Creeds to judge what an “orthodox Christian” is to believe, he is using
the same standard that Catholics, Orthodox, all the Fathers, Saints, and Doctors have used in the visible Catholic Church for 2000 years. A little Philip Hughes for you:
“The first General Council met in 325. The Church had then been an established fact for nearly three hundred years…the idea that what bishops collectively agree is law has a binding force that is greater than any of their individual instructions to their own see. To begin with the last point, it is a safe statement that
from the moment when history first shows us the Church of Christ as an institution, the exclusive right of the Church to state with finality what should be believed as Christ’s teaching is manifestly taken for granted. To bring out a theory of belief, or to propose a change in morals which conflicts with what the Church universally holds is, from the very beginning,
to put oneself fatally in the wrong. The immediate, spontaneous reaction of the Church to condemn thinkers with new and original views of this kind is perhaps the most general, as it is the most striking, of all the phenomena of the Church’s early history, so far back as the record goes.” (Philip Hughes,
A History of the General Councils, introduction)
When Slick says “Catholics are not Christians” because they do not hold to a
16th-century interpretation of Justification by Faith Alone, he is using a standard for “Christianity” that was invented in the 16th century.

Very simple. It’s a matter of history.
“The significance of the Protestant distinction between -iustificatio- and -regeneratio- is that
a fundamental discontinuity has been introduced into the western theological tradition where none had existed before…the essential point is that a notional distinction is made
where none had been acknowledged before in the history of Christian doctrine…A fundamental discontinuity was introduced into the western theological tradition
where none had ever existed, or ever been contemplated, before. The Reformation understanding of the nature of justification – as opposed to its mode – must therefore be regarded as a genuine theological novum.” (Anglican evangelical scholar Alister McGrath,
IUSTITIA DEI: A History of the Christian Doctrine of Justification, Volume 1, Chapter 5, Section 19 conclusion)
The next question we (Mormons and Catholics) have is why do the Creeds have the authority Catholics (and Orthodox and most Protestants) give to them?
Because that authority goes back to the authority of Christ and there was no “apostasy” from the teachings of Christ. But on that point, Mormons and Catholics disagree.