The existence of human messengers does not imply that all who claim to have a message from God actually do. There are people who are actuated by the devil, but claim to be inspired from God (see Alma 30:53). There are people who are con artists (short for convincing artists), who personally benefit from manipulating people to do as they say. There are people who are sincere, but of limited understanding, whose zeal exceeds their knowledge (see Romans 10:2). An apt example of this is Apollos, who went around preaching and teaching the baptism of John. He wasn’t preaching about Jesus only because he hadn’t yet heard of him (see Acts 18:24-26).
You won’t know which type of person you are dealing with until you test what they are saying. People with legitimate light and truth will be the minority.
There is a difference between a deceived prophet and a false prophet. Both exist. A false prophet never came from God. A false prophet is a total phony. False prophets are liars, and they know they are liars. A deceived prophet is different. A deceived prophet may have accessed the heavens. He may have had many interactions with God. He may have worked true, mighty wonders from heaven. Yet, none of these things prevents a man from teaching false doctrine, from being mistaken, or from teaching on a topic beyond his understanding. Prophets only know what God has shown them, and no prophet has seen everything. Like anyone else, prophets must synthesize what God has shown them with their understanding of everything else. Everything else could come from their understanding of the scriptures, studies of human behavior, cultural biases, and what they have heard from others.
Furthermore, all men are capable of possessing lesser truth. For example, consider John the Baptist. He preached a higher degree of truth than the Jews, but a lower degree of truth than Christ. Christ's teachings would not have been new or radical if they were simply the same things as John preached. They were not. Some followers of John recognized that Jesus had higher truth, and started listening to Jesus instead of John. Many did not. While post-Jesus followers of John (such as the disciples Paul encountered baptizing with the baptism of John) were not preaching false doctrine, they were certainly preaching doctrine of lesser light than the followers of Jesus (see Acts 19). Those who cling to lower light when shown higher light are akin to men who opt to eat stale, moldy bread when hot, fresh bread is offered to them.
This works the other way as well. Truth can come from anywhere, including from people who have previously done things that are clearly bad. The high priest who sentenced Jesus to death also gave a true prophecy:
49 And one of them, named Caiaphas, being the high priest that same year, said unto them, Ye know nothing at all,
50 Nor consider that it is expedient for us, that one man should die for the people, and that the whole nation perish not.
51 And this spake he not of himself: but being high priest that year, he prophesied that Jesus should die for that nation. (John 11)
Rejecting a principle for where it came from is perhaps the greatest form of unbelief. Most people mistakenly think they are testing a principle when in fact they are merely testing where it came from. The truthfulness of a principle has nothing to do with where it came from. It is by their fruits that ye shall know them. Not the trunk of the tree.
The source of a message does not impact its truthfulness.
No man is perfect. Given that truth, the search for deeper understanding of gospel principles will always include uncovering the mistakes, misconceptions, and limitations of understanding in others. This is completely natural given the process of seeking for truth. As referenced in another chapter, consider the analogy of a city at night. With various sources of light at different intensities, the truth seeker has an ever-changing snapshot of all truth currently understood by them. Some parts of this snapshot are more certain than others. For an outsider to see another’s viewpoint that is not in concordance with his own, then, is the norm and not the exception. Mistakes are simply a side effect of the process, just as dross is a byproduct of deriving metal from ore. The dross of others’ mistakes is not what truth-seekers are after. They will simply cast it aside in their search for the ore of personal improvement.
Past performance is not a validator of truth. A person can be totally right in the past, and totally wrong in the present, or vice versa. Every message must be independently evaluated: “...test everything; hold fast what is good.” (1 Thessalonians 5:21) One part of a sermon can be correct, even if another is not.