My response to believing that what Joseph Smith said was true is as follows:
I believe that there are truths in many religions and philosophies, but that does not make that religion or philosophy the Truth. JS said a lot of things that rang true for a group of people at a specific place and time in history. Forgive me, but so did Hitler, David Koresh, Jim Jones, L. Ron Hubbard…and 175 years from now there may be people who still believe that these men spoke The Truth because SOME of what they said rings true. Furthermore, the fact that some of their followers were, or are martyrs is still not enough to make their religion or philosophy true. This is so, no matter how embedded they are into popular culture, contemporary thoughts and values. My examples of erroneous leaders are extreme but the less obviously flawed founders tend more to muddle our thinking.
What I think many Catholics are expressing here on this forum is that our faith was founded on the Truth. Imperfect humans struggled and prayed to understand that Truth and live it (and still do struggle and pray). While the Church recognizes private revelation, our faith does not depend on it and,in most cases, we are highly skeptical. (Please refer to above list: “Hitler, Koresh”, etc.) Not all revelation is divine AND, in fact, over time such revelations can lead people far from the Truth.
For example, misunderstandings and falsehoods about the Virgin Mary can lead a lot of souls to hell. And by hell I mean both the eternal hell as well as the hell we now live in where we “pick and choose” our morality, depending on the weather or the latest Dan Brown or Steve Berry “book of revelations”.
I love my mormon and protestant friends and their fervency yet I was guided back to the Catholic church based on the quote attributed to Jackie Gleason: “I would rather be a bad Catholic than a good anything else.” I chose to struggle with the Truth expressed in the Catholic Church through centuries rather than feel comfortable with some of the easier," light" doctrines of other more recent sects. When I have an occasional moment of private revelation, I reach out to a catechism, priest, or scholar for consultation in order to be sure I am not heading too far off the highway into ImaginationLand. The responsibility is on me to drive well and follow the laws while moving closer to union with God; the pressure is off me having to learn or unlearn someone’s latest prophecy, revelation, or doctrine.