Hugh Nibley
"The regular scriptural term to describe the leaders of all unauthorized congregations is false prophets. The fatal defect of such congregations is that they are led by false prophets, and we are told that these would abound in the earth, all claiming to be followers of Christ.
"What is a false prophet? He is one who usurps the prerogatives and the authority which by right belong only to a prophet of God. The false prophet need not claim to be a prophet; indeed, most false prophets do not believe in prophecy or even in God, nor do they want anyone else to…we still live in a world of false prophets. Anyone whose work competes with God's work, who makes claims on the time and energies of men which rightly belong to God, who puts the word of God in second place to the theories of men, or forces the teachings of true prophets to yield precedence to his own discourses--anyone, in a word, who puts his own knowledge above or on a level with revelation from heaven is a false prophet." (The World and the Prophets, pp. 254-5)
Joseph Smith
"If any person should ask me if I were a prophet, I should not deny it, as that would give me the lie; for, according to John, the testimony of Jesus is the spirit of prophecy; therefore, if I profess to be a witness or teacher, and have not the spirit of prophecy, which is the testimony of Jesus, I must be a false witness; but if I be a true teacher and witness, I must possess the spirit of prophecy, and that constitutes a prophet; and any man who says he is a teacher or a preacher of righteousness, and denies the spirit of prophecy, is a liar, and the truth is not in him; and by this key false teachers and impostors may be detected." (Teachings of the Prophet Joseph Smith, p. 269)
Joseph Smith
"False prophets always arise to oppose the true prophets and they will prophesy so very near the truth that they will deceive almost the very chosen ones." (Teachings of the Prophet Joseph Smith, p. 365.)
J. Reuben Clark, Jr.
"The ravening wolves are amongst us, from our own membership, and they, more than any others, are clothed in sheep's clothing, because they wear the habiliments of the priesthood; they are they [who are] distorting the truth. We should be careful of them." (Conference Report, Apr. 1949, p. 163)
Neal A. Maxwell
"Following the Brethren can be more difficult when in some settings wolves are sent among the flock. False prophets will arise, enticing some to follow them, and by their evil works they deceive careless observers into discounting any and all who claim to be prophets. Satan's order of battle is such that if it is necessary to encourage a hundred false prophets in order to obscure the validity of one true prophet, he will gladly do so." (All These Things Shall Give Thee Experience, p. 115)