Basically, the problem we have, Tony et al Mormons, is that Mormonism is based primarily on the private revelation of one man, with few or no witnesses to verify his statements.
Christianity proper is based on the very public historical events of Christ’s crucifixion, Resurrection, and wandering about Palestine for 40 days after His Resurrection. Thousands saw Him wander the hills of Israel after His Death and Rising. They saw His wounds. They saw His miracles. It was more than one or two family members; it was all eleven of His Apostles, His Mother, and the whole bloody city of Jerusalem - which was even more packed with people because it was Passover.
Jesus’s Death and Resurrection is too public, and at the time was too well-known, to be a sham. The Apostles would have been laughed out of Jerusalem, and/or stoned to death.
While we are not so strict with heretics anymore, Joseph Smith’s “revelations” were not at all public. At best he had one, maybe two family members to testify to his “visions” and “miracles” (correct me if I am wrong). Whatever the case, there is almost no way to know if Joseph Smith’s visions actually happened.
But, fictitious or not, we can know if Smith’s accounts contradict Christ’s Gospel (upon which Mormonism is built): First, Joseph Smith and early Mormons preached total apostasy - that, according to Jesus, all the other churches were errant and not the True Church.
But this contradicts with Matthew 16:19 and Matthew 28:20 (any other Scriptures? Help me out, guys!), where Christ says that the gates of Hell will not prevail against the Church, and He will be with us to the end of time. How can Christ be in a Church where heresy exists? One lost thread unravels the whole blanket (eventually). If all churches before Smith were heretical, then Christ was not with us (at least not completely), and Hell did prevail against Christ’s Church.
Also, if every Church was in heresy, why does Smith 1) use the Protestant canon of Scripture (aside from the BoM, the PoGP, etc) , and not his own, and 2) claim apostolic descent? Of what value is apostolic descent if their church did not even survive?