Allow me to explain. A very popular passage in the Book of Mormon, Alma 32:28-33, compares the gospel to a seed. The passage is as follows:
28 Now, we will compare the word unto a seed. Now, if ye give place, that a seed may be planted in your heart, behold, if it be a true seed, or a good seed, if ye do not cast it out by your unbelief, that ye will resist the Spirit of the Lord, behold, it will begin to swell within your breasts; and when you feel these swelling motions, ye will begin to say within yourselves—It must needs be that this is a good seed, or that the word is good, for it beginneth to enlarge my soul; yea, it beginneth to enlighten my funderstanding, yea, it beginneth to be delicious to me.
29 Now behold, would not this increase your faith? I say unto you, Yea; nevertheless it hath not grown up to a perfect knowledge.
30 But behold, as the seed swelleth, and sprouteth, and beginneth to grow, then you must needs say that the seed is good; for behold it swelleth, and sprouteth, and beginneth to grow. And now, behold, will not this strengthen your faith? Yea, it will strengthen your faith: for ye will say I know that this is a good seed; for behold it sprouteth and beginneth to grow.
31 And now, behold, are ye sure that this is a good seed? I say unto you, Yea; for every seed bringeth forth unto its own alikeness.
32 Therefore, if a seed groweth it is good, but if it groweth not, behold it is not good, therefore it is cast away.
33 And now, behold, because ye have tried the experiment, and planted the seed, and it swelleth and sprouteth, and beginneth to grow, ye must needs know that the seed is good.
This passage essentially means that if you try out the gospel by living it, you will experience good effects in your life, and that will tell that the gospel is good (and, therefore, true).
There's one problem with this, though. It embodies a logical fallacy called affirming the consequent, which looks like this:
If A, then B.
For example, suppose the following:
If it is raining, then the sidewalk is wet.
The sidewalk is wet.
Therefore, it is raining.
Obviously, that structure doesn't work. There are plenty of reasons why the sidewalk could be wet besides rain; maybe your neighbor has the sprinklers on. So, let's put Alma 32 into a syllogistic format:
If the gospel is true (the seed is good), then you will experience positive effects in your life when living the gospel (planting the seed and it begins to grow)
You experienced good effects
Therefore, the gospel is true
To make it even more straightforward, let's use letters instead. P = experiencing positive effects after trying the gospel (planting the seed and it begins to grow), and T = the gospel is true (it is a good seed). It looks like this:
If T, then P
This is a clear case of affirming the consequent, and therefore fallacious.