An ad hominem fallacy is one where, instead of attacking an argument directly, you attack a person making the argument, and use your attacks on that person as a way to discredit the argument. In short, you’re ignoring the thing being debated and attacking the debater.
Many of those who have become disaffected from the LDS church are guilty of ad hominem fallacies. They decry that since the church is corrupt, its message must also be corrupt. Similarly, those in the church are guilty of this whenever they slap the label “anti-Mormon” on something and use that as an excuse to not listen.
Ad hominem also works in the inverse, too. The conventional meaning is that you distrust something because of where it comes from, but the inverse is also a fallacy: to give credit to something because of its source, and not because of the thing itself. Mormons do this a lot in the form of the “Follow The Prophet” trope. “Anything that the Prophet says is true because the Prophet said it” is an ad hominem fallacy. Similarly, for some disaffected Mormons, “Anything that Denver Snuffer says is true because Denver Snuffer said it.”
In both of these cases, the important thing is being ignored, and we are focusing on something that does not really matter. In both cases, the source of the material is completely irrelevant. It does not matter who said it. What matters is what was said. Is the message of God? Does it encourage you to believe in Him and deepen your personal relationship with Him? Or does it drive you away? Does it entice you to scorn your brothers and sisters and place yourself (or anyone else) on a pedestal?
We can extend this to other things as well. People dismiss the Book of Mormon because they believe Joseph Smith was a fraud. This is an ad hominem fallacy. The Book of Mormon can be true and important and relevant regardless of how it came in to being. It does not matter if it came from golden plates divinely translated by an uneducated farm boy, or if it came from the dictations of a white salamander, or it if came from the machinations of conspiring men. What matters is what it teaches.
All humans are fallacious. We are all weak, culpable, fragile, and unreliable beings. But that does not stop us from being capable of beautiful works and great goodness. To ignore or reject something good simply because you don’t like where it comes from is wrong. And accepting something evil because you like the source is equally wrong.
The sources don’t matter. Just the message.