Question: How can we tell if a modern revelation comes from God or Satan?

Moroni: "that which is of God inviteth and enticeth to do good continually; wherefore, every thing which inviteth and enticeth to do good, and to love God, and to serve him, is inspired of God"

But behold, that which is of God inviteth and enticeth to do good continually; wherefore, every thing which inviteth and enticeth to do good, and to love God, and to serve him, is inspired of God.

Philosophy

How do we know the world is not an illusion? How do we know we are not just a brain in a vat? How do we know our life is not a dream?

We just don't believe any of that, we just take what our senses tell us. Such questions are just meaningless.

Consider what Joseph Smith told Brigham Young

Tell the brethren to be humble and faithful and be sure to keep the Spirit of the Lord, that it will lead them aright. Be careful and not turn away the still, small voice; it will teach them what to do and where to go; it will yield the fruits of the kingdom. Tell the brethren to keep their heart open to conviction, so that when the Holy Ghost comes to them their hearts will be ready to receive it. They can tell the Spirit of the Lord from all other spirits—it will whisper peace and joy to their souls; it will take malice, hatred, strife and all evil from their hearts, and their whole desire will be to do good” (quoted in Juvenile Instructor, 19 July 1873, 114)

Prophet Gordon B. Hinckley said:

That’s the test, when all is said and done. Does it persuade one to do good, to rise, to stand tall, to do the right thing, to be kind, to be generous? Then it is of the Spirit of God. . . . If it invites to do good, it is of God. If it inviteth to do evil, it is of the devil. . . . And if you are doing the right thing and if you are living the right way, you will know in your heart what the Spirit is saying to you.
You recognize the promptings of the Spirit by the fruits of the Spirit—that which enlighteneth, that which buildeth up, that which is positive and affirmative and uplifting and leads us to better thoughts and better words and better deeds is of the Spirit of God” (Teachings of Gordon B. Hinckley, 260–61)

Elder Dallin H. Oaks stated:

[A person may have] a strong desire to be led by the Spirit of the Lord but . . . unwisely extends that desire to the point of wanting to be led in all things. A desire to be led by the Lord is a strength, but it needs to be accompanied by an understanding that our Heavenly Father leaves many decisions for our personal choices. Personal decision making is one of the sources of the growth we are meant to experience in mortality. Persons who try to shift all decision making to the Lord and plead for revelation in every choice will soon find circumstances in which they pray for guidance and don't receive it. For example, this is likely to occur in those numerous circumstances in which the choices are trivial or either choice is acceptable.
We should study things out in our minds, using the reasoning powers our Creator has placed within us. Then we should pray for guidance and act upon it if we receive it. If we do not receive guidance, we should act upon our best judgment. Persons who persist in seeking revelatory guidance on subjects on which the Lord has not chosen to direct us may concoct an answer out of their own fantasy or bias, or they may even receive an answer through the medium of "false revelation" (“Our Strengths Can Become Our Downfall,” Ensign, Oct. 1994, 13–14)

This statement can apply to everyone, even to General Authorities as a voice of warning

President Howard W. Hunter said

“Let me offer a word of caution. . . . I think if we are not careful . . . , we may begin to try to counterfeit the true influence of the Spirit of the Lord by unworthy and manipulative means. I get concerned when it appears that strong emotion or free-flowing tears are equated with the presence of the Spirit. Certainly the Spirit of the Lord can bring strong emotional feelings, including tears, but that outward manifestation ought not to be confused with the presence of the Spirit itself” (The Teachings of Howard W. Hunter, 184)

Preach My Gospel: A Guide to Missionary Service, "How Do I Recognize and Understand the Spirit?"

The Church of Jesus Christ of Latter-day Saints, Preach My Gospel: A Guide to Missionary Service, (2004)

The Spirit is always available to guide and direct you. However, the Spirit speaks quietly, through your feelings as well as your mind. One great challenge for you and those you work with is to recognize the quiet, subtle promptings of the Holy Ghost.

President Boyd K. Packer taught: “The voice of the Spirit is described in the scripture as being neither ‘loud’ nor ‘harsh.’ It is ‘not a voice of thunder, neither … voice of a great tumultuous noise.’ But rather, ‘a still voice of perfect mildness, as if it had been a whisper,’ and it can ‘pierce even to the very soul’ and ‘cause [the heart] to burn.’ (3 Ne. 11:3; Hel. 5:30; D&C 85:6–7.) Remember, Elijah found the voice of the Lord was not in the wind, nor in the earthquake, nor in the fire, but was a ‘still small voice.’ (1 Kgs. 19:12.)
“The Spirit does not get our attention by shouting or shaking us with a heavy hand. Rather it whispers. It caresses so gently that if we are preoccupied we may not feel it at all. (No wonder that the Word of Wisdom was revealed to us, for how could the drunkard or the addict feel such a voice?)

“Occasionally it will press just firmly enough for us to pay heed. But most of the time, if we do not heed the gentle feeling, the Spirit will withdraw and wait until we come seeking and listening and say in our manner and expression, like Samuel of ancient times, ‘Speak [Lord], for thy servant heareth.’ (1 Sam. 3:10.)” (“The Candle of the Lord,” Ensign, Jan. 1983, 53).

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Notes

Question: How can we tell if a modern revelation comes from God or Satan?