Get them to voice the two important disclaimers about their test.
“What if I pray sincerely and don't get an experience/feeling?”
You want them to admit that it feels different for different people. So ask:
“What will it feel like? Is it a feeling of peace, excitement, or something else?”
Express your very understandable concerns about their test.“Bad tests yield bad results”: Comment that this test “worries” you; it is worth raising an eyebrow when a religion devises its own way to test itself. For example, say…
“…I know Muslims insist that we can know Islam is true by reading the Qur'an and asking ourselves whether it is not the most beautiful work in existence—we might call it the read-feel test. If we judge the Qur'an as sufficiently beautiful, then this allegedly confirms the truth of Islam. If you come out not being convinced, however, you need to read it again and/or consider more deeply and sincerely. If this read-feel was proposed to me by a Muslim, I would say two things:
—First, I see no reason to trust this test.
—Second, I see reason to not trust this test; there are reasons to think it will mislead. (If people want to believe it badly enough, they will convince themselves that the Qur'an is the most beautiful.)
I have similar concerns about the pray-feel test.”
Let's explore these two concerns in turn.
“I see no reason to trust this test”: Regarding your first concern, communicate the worry that this test is expecting God to jump through hoops of our own making. E.g. …
“…doesn't expecting God to participate in this test kind of reminds you of people who reject medical treatment because they expect God to save them? They try to say that this is something God would do, after all, because God loves us, which I guess sounds kind of pious at first, but in the end, unless it was promised by God, aren't they imposing their own expectations on God? Aren't they trying to make God jump through hoops they have set up for him? Isn't that worldly wisdom? Why think God will participate in this test we are told to trust in? It might be worth studying what the Bible says about testing prophets and their new claims.”
Often, Mormons will cite James 1:5. If they do, there is simple objection you can raise without sounding too much like a combative apologist. Read it with them again very slowly—it merely says “If any of you lacks WISDOM, let him ask of God.” You can gently point out that (a) this is not a test (they were not trying to figure out of something was true), and that (b) wisdom and knowledge are very different. You might say
“Sure, I've never had a problem praying things like 'Dear God, obviously, if Mormonism is true, I'd like to know it. Please grant me patience, wisdom, and other important virtues in my investigation.' But that's not a test, right? Wisdom a more like the proper application of knowledge. This is not the Bible telling us how to test a prophet or anything.”
“I see reasons to not trust this test”: There are a number of indicators that the feelings which Mormons attribute to the Holy Spirit would occur even if the Holy Spirit were not involved. You can say something like this…
“But I will look in to it”: After expressing these preliminary concerns, you might say…“Here is my other worry… which maybe you can help with. It seems to me that if the Holy Spirit were not involved, and people were just tricking themselves, that you would still have people regularly saying the Holy Spirit was involved. Right? After all,
—if you get any positive feeling, you can convince yourself that it was the Holy Spirit, which explains why the feeling is allegedly different for different people. Right?
—if you get no feeling, then you just keep trying until you get a feeling. Why think you wouldn't get a feeling eventually? Especially if you really want one?
I mean, it seems like this is kind of what we'd expect to see if there were no Holy Spirit involved, and people were just tricking themselves. That's why I was saying it reminded me of the Islam test. I'm not trying to be difficult or anything, that's just how it comes off to me.”
“…I don't know, maybe I'm being paranoid. I don't think I am, but I'll need to think about this more and research it a bit. I just want to be careful, that's all. I will pray for wisdom and guidance from God.”
They are likely going to compliment you on your wisdom and honesty. So now you have presented all your wisdom-based objections, and you have prepared them to hear out your knowledge-based objections to the test when next you meet.
Knowledge-Based Objections to the Pray-Feel Test
“Ok, I spent some time looking in to this, and I came across some interesting objections I wanted to run by you. I don't know if they areany good, but they seemed convincing enough to me.”
Your bread-and-butter objection is this:
“Jesus warned that many false prophets would come (Mt 7:15), and early Christians were on high alert (2 Cor 11:4-15; Gal 1:6-9; 1 Tim 4:1; 2 Pet 2:1-3; 1 Jn 4:1; Jude 3-16). It turns out that Bible is overflowing with examples where a Godly individual, be it Moses or the apostles, wanted to know whether something was true. They consistently…The pattern seems to be that we recognize new prophet or revelation as divine when (a) it is authenticated by evidence suggesting its origin is supernatural, notably miracles, and (b) the content of the revelation stands undefeated. That is to say, we lack reason to think it is false. If new revelation contradicts previous revelation, that is one way in which it can fail criterion (b). You can see this test being championed in multiple locations. For example:
Acts 17:11 -- Now these were more noble-minded than those in Thessalonica, for they received the word with great eagerness, examining the Scriptures daily to see whether these things were so.
Similarly, here is another way it can fail criterion (b):
Deuteronomy 18:21-22 (KJV) -- if thou say in thine heart, How shall we know the word which the LORD hath not spoken? When a prophet speaketh in the name of the LORD, if the thing follow not, nor come to pass, that is the thing which the LORD hath not spoken, but the prophet hath spoken it presumptuously: thou shalt not be afraid of him.
So, test a prophet objectively, by his fruit. Not once in all the Biblical examples where a prophet or message is being tested did a good character employ anything even remotely resembling the pray-feel test. Deviating from the Biblical format of testing is just inviting yourself to be mislead; it is precisely the kind of thing we would expect of a false prophet or group (or worse) to advocate when they can't pass the real test advanced in scripture. The Bible is God's word, and Mormonism claims to have additions. If the additions are true, then they should be able to stand the tests recommended in what we already know to be God's word.