For those who are LDS, one of those key questions is whether the early Church apostasized. If it didn’t, does that mean Mormonism is totally wrong? Of course not. Personally, I am willing to accept the theory that Joseph Smith received some true, and some false, revelations from God. The final test is the Catholic Church. What he taught that was true I’m willing to accept, what he taught that was false, I must reject. Such a theory is far from implausible, because we know that all people are a combination of truth and error. And specifically if a person has a prophetic gift (as I believe Smith had to a certain degree), they are going to get some things correct, and get some things wrong, as the flesh and the devil gets in the way.
How do we know what is true and what is in error? Do we simply trust what the person says? No, Sacred Scripture tells us how we know the “spirit of truth” and “the spirit of error.”
1 John 4:6: “We are of God: he that knoweth God heareth us; he that is not of God heareth not us. Hereby know we the spirit of truth, and the spirit of error.”
St. John is saying here that we (Christians) can always know what is true and what is erroneous by seeing if any teaching “listens to the apostles” i.e. conforms to their teaching. How can we do such a thing if the Church apostasized and the apostolic teaching was lost?