If you talk with Mormon missionaries for any length of time, at some point, they are going to instruct you to read the Book of Mormon and to pray for a direct revelation of God that the book is true. They claim that, in response to this prayer, you will receive good feelings, and those good feelings are the voice of God confirming the book (and, by extension, their whole religion). Mormon scriptures teach this in various places, such as:
"But behold, if ye will awake and arouse your faculties, even to an experiment upon my words, and exercise a particle of faith, yea, even if ye can no more than desire to believe, let this desire work in you, even until ye believe in a manner that ye can give place for a portion of my words. 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 understanding, yea, it beginneth to be delicious to me," (Alma 32:27-28).
Mormon's often mock Christians who caution against this test of "praying about the Book of Mormon," claiming that these cautions prove that such Christians aren't really trusting in God. "What could possibly be wrong with praying about something? Why are you afraid of prayer? Isn't prayer always a good thing? If people are cautioning you against prayer, doesn't that prove that their teaching isn't of God?" etc. But, of course, such Christians are not saying to stop praying. They are saying that it is wrong to ignore what God has already plainly said and to presumptuously pray for your own personal revelation on something to which God has already spoken. They are also pointing out that our inner feelings are untrustworthy and should not be confused with the voice of God, and to offer up a prayer with the intention of then trusting and even deifying our feelings is wrong. But even on the more basic level, the Mormon assumption that God would never command us not to pray about something is simply untrue! For example, God repeatedly said to the prophet Jeremiah things like:
"As for you, do not pray for this people, and do not lift up cry or prayer for them, and do not intercede with Me; for I do not hear you," (Jeremiah 7:16, see also 11:14 and 14:11).
In the New Testament, there are likewise people for whom we are actually instructed not to pray, (1 John 5:16). So one cannot simply assume that just because prayer is a good thing that therefore there is nothing about which we should not pray. And when it comes to matters about which God has already spoken and already instructed how we are to handle them, is it not wrong to go to God and say, "never mind your previous instructions! Tell me this now in a new way, personally, individually, through the specific subjective inner sign that I am asking for!" When people demanded a sign from Jesus, He was not generally forthcoming. Indeed, we are cautioned against praying to God about things that He has already answered.
Remember the story of Balaam. The nobles of Moab came from the king and promised rich reward for Balaam if he would come and Curse Israel, but the LORD strictly forbid Balaam to go. The king sent greater nobles and promised far greater riches. Balaam had his answer already. God had already spoken, yet Balaam decided it couldn't hurt to pray about it anyway. God permitted Balaam to go, but in fact was angry with Balaam. Balaam had sinned by seeking an answer contrary to what God had already spoken, (Numbers 22:1-22). It was wrong for Balaam to ask God in this circumstance, and when Balaam did ask, God gave Balaam over to his own errant desire, ultimately to Balaam's own destruction. When we ought to already know what we should do, God is not pleased in our prayer. As when Moses prayed while Israel was perused by the armies of Egypt and God replied:
"Why are you crying out to Me? Tell the sons of Israel to go forward," (Exodus 14:15).
When it comes to those who claim to be prophets and to offer new revelations from God, we have already been told what to do. Jesus said:
"Beware of the false prophets, who come to you in sheep’s clothing, but inwardly are ravenous wolves. You will know them by their fruits. Grapes are not gathered from thorn bushes nor figs from thistles, are they? So every good tree bears good fruit, but the bad tree bears bad fruit. A good tree cannot produce bad fruit, nor can a bad tree produce good fruit. Every tree that does not bear good fruit is cut down and thrown into the fire. So then, you will know them by their fruits," (Matthew 7:15-20).
Jesus did not say to pray about the new prophet and see if our bosom burns or feel wonderful swelling motions in our chest or any other inner sensation. No, Jesus told us to examine their fruit. Now, many Mormons will actually bring up this passage and boast of their church's great financial giving and their supposed charity and generosity and say, "look our fruit! See, how can our church not be true when they give away so much so publically!" Ironically, earlier in the very same sermon, Jesus points out that boasting openly about one's charity is actually a sign of hypocrisy! (Matthew 6:2). Giving is, instead, to be done in secret and known only to God, (Matthew 6:3). Public charitable contributions are not the kind of fruit Jesus is talking about. Neither are public acts of piety like prayer and fasting. The hypocrites do these very things for all to see. Such is the sheep's clothing, not the fruit. In the parallel passage in Luke, Jesus says:
"For there is no good tree which produces bad fruit, nor, on the other hand, a bad tree which produces good fruit. For each tree is known by its own fruit. For men do not gather figs from thorns, nor do they pick grapes from a briar bush. The good man out of the good treasure of his heart brings forth what is good; and the evil man out of the evil treasure brings forth what is evil; for his mouth speaks from that which fills his heart," (Luke 6:43-45).
And later in Matthew, Jesus elaborates:
"Either make the tree good and its fruit good, or make the tree bad and its fruit bad; for the tree is known by its fruit. You brood of vipers, how can you, being evil, speak what is good? For the mouth speaks out of that which fills the heart. The good man brings out of his good treasure what is good; and the evil man brings out of his evil treasure what is evil. But I tell you that every careless word that people speak, they shall give an accounting for it in the day of judgment. For by your words you will be justified, and by your words you will be condemned,” (Matthew 12:33-36).
The fruit is what comes out of the mouth. The fruit of the prophet is the prophet's words. We discern a false prophet by ignoring their outer works and looking into the heart through what they say and what they teach. We evaluate them by their doctrine and prophecy. Paul later warns similarly:
"But even if we, or an angel from heaven, should preach to you a gospel contrary to what we have preached to you, he is to be accursed! As we have said before, so I say again now, if any man is preaching to you a gospel contrary to what you received, he is to be accursed!" (Galatians 1:8-9).
Our Mormon Missionary friends tell us that Joseph Smith received his gospel from angels and great visions. They further claim that the Apostles of Jesus came down from heaven and endowed him with their authority. Paul warns us that, even if this were true, none of it matters if he preached a different gospel. Even angels, even apostles like Paul himself, are to be rejected if they preach a different gospel.
So when the Mormon (or anyone else) comes to us with a new teaching, we are to follow the commands and instructions we have already been given, We are to evaluate their teachings based on what God has already revealed, and we are to reject any message that is contrary and any other gospel. If we ignore our Lord's warning and instead pray that God would just give us happy feelings so we can believe this new prophecy without having to examine it critically, God may just do to us as He did to Balaam and give us over to our desire. No, let us hold fast to the words that our God has already spoken. Prayer in disobedience is not virtuous. Hold fast and stand firm on that which has already been revealed and is known to be true. Accept no counterfeits.