I understand based on previous threads that the Mormon Church does not accept the ancient ecumenical councils, which also influences how we all look at this matter. Not sure how to broach that, since it seems mormons consider them inaccurate while we consider them the foundations of our theology and our understanding of scripture.
That is indeed correct, but it's where faith comes in. Joseph Smith was having trouble deciding which church to join, so he went to a secluded grove and prayed, honestly and fervently, and was surprised when Heavenly Father and Jesus Christ both appeared to him - not in a vision, but in person! Joseph asked his question about which church was the correct one, and Jesus told him "none of them." None of the churches had the fulness of His gospel.
It's certainly a controversial belief - especially for those outside the LDS faith - but it's one of the cornerstones. That Joseph Smith did have a personal visitation from those two individual entities in the "sacred grove" in New York State, and had the communication that would eventually lead to the discovery of the golden plates and the Book of Mormon. It's understandable why other churches would reject this belief (and, in fact, they did). But millions of people have received a witness from the Holy Spirit that this did, indeed, happen, myself included.
Whether or not you believe it depends on you and your communications with the Holy Spirit. But it gives you an idea as to where the Church of Latter-day Saints is coming from, at least.
(Incidentally, Joseph Smith's story really resonated with me because I had a similar experience. Many, many years ago, before I met my wife, there was a girl I met at college who was a member of a Pentecostal church. And, of course, I liked her. After watching a baptism at their church, I went to my room and prayed on my knees, asking if I should join this church. I received a clear message of "no," and an additional message - "be wary of any church that puts more emphasis on ritual, rather than the meaning behind them." To make a long story short, many years later, when I asked if I should join the LDS church, I got the overwhelming response of "yes." In various formats. I didn't understand why - especially given my prior message - but I didn't want to tell Heavenly Father "no," so....)