Mormons on the Fruit of Matthew 7

I had some Mormons come over and visit me. And unfortunately, my experiences with the Mormons are too similar. I show them that the Book of Abraham is a fraud, and thus makes the Book of Mormon very suspicious, and they ignore evidence and rely on “faith”. Faith, in spite of evidence. They get angry, and I don’t blame them, I’m attacking such a central part of their lives, of course they’re going to get defensive, but it’s such a diabolical doctrine, it needs to be weeded out.

After grasping for straws, they referenced Matthew 7:15-20, and said you can tell which church is the true church by its fruit, or whether or not it’s fruitful. My typical response to this is to apply a reductio and say then they have to cede that other religions are true by this test as well. Their response is that I’m saying Jesus is a liar, and I respond again by saying their interpretation of that scripture is false. This is a lot of back and forth, and so now I think I have a better way of handling this.

First, we need to know a little bit of logic; in this case, modus ponens. Basically, modus ponens is an “If…then…” statement. “If it is Tuesday, then I will go to work.” “If it is raining, then the ground is wet.” “If I am Adrian Urias, then I am a male.” “If I’m saved, then I was baptized.” Formally, it will look like:

If A, then B
A
Therefore, B

So lets take a look at the examples I just gave, and apply modus ponens.

If it is Tuesday, I will go to work
It is Tuesday
Therefore, I will go to work

If it is raining, then the ground is wet
It is raining
Therefore, the ground is wet

If I am Adrian Urias, then I am a male
I am Adrian Urias
Therefore, I am a male

If I’m saved, then I was baptized
I’m saved
Therefore, I was baptized

These are all valid arguments. Then there is Modus Tollens, which denies the consequent. Formally, it looks like:

If A, then B
Not B
Therefore, Not A

So it would look like:

If it is Tuesday, I will go to work
I will not go to work
Therefore, it is not Tuesday

If it is raining, then the ground is wet
The ground is not wet
Therefore, it is not raining

If I am Adrian Urias, then I am a male
I am not a male
Therefore, I am not Adrian Urias

If I’m saved, then I was baptized
I was not baptized
Therefore, I am not saved

These are all valid arguments. However, when you affirm the consequent, then your argument is invalid. Formally, it would look like:

If A, then B
B
Therefore, A

This is fallacious. We can see it when applied.

If it is Tuesday, I will go to work
I will go to work
Therefore, it is Tuesday
This is wrong because Tuesday might not be the only day I have to go to work.

If it is raining outside, then the ground is wet
The ground is wet
Therefore, it is raining outside
This is incorrect because the ground could be wet for other reasons, like a sprinkler going off.

If I am Adrian Urias, then I am a male
I am a male
Therefore, I am Adrian Urias
Obviously, not all males are Adrian Urias. Though most should be.

If I’m saved, then I was baptized
I was baptized
Therefore, I am saved
A baptized person could have fallen away. Once saved, doesn’t mean always saved.

Now then, let’s return to the claim of the Mormon. According to their interpretation of Matthew, the true church will reveal itself by its fruit. So if it’s the true church, then it will have good fruit. And because they’ve done so many good works, and made people feel good, they must be the true church.

If (they are the true church), then (they will have good fruit)
If               (A )                                , then                      (B)
(They have good fruit)
(B)
Therefore, (they are the true church)
Therefore,                    (A)

This is an invalid argument. They are affirming the consequent, which is a basic no-no in logic. There are other points you can bring up when talking about Matthew 7, but this may be the quickest way. One point you can bring up is ask them what fruit means. Does it just mean your external actions? Your works? If so, then that seems like a morally deficient way of detecting a true church. What if someone donates money to a hospital, not out of sincere concern for the patients, but merely to look good in the community? What kind of fruit is this person producing? Or what about those atheists who do some really nice things that put some Christians to shame? Are they part of the right church, even though the flat out deny God?

Good fruit is necessary, but it is not sufficient to be a Christian. All Christians have good fruit, but not everyone who has good fruit is a Christian (All A’s are B’s, but not all B’s are A’s).