"For such are false apostles, deceitful workers, transforming themselves into the apostles of Christ. And no marvel; for Satan himself is transformed into an angel of light. Therefore it is no great thing if his ministers also be transformed as the ministers of righteousness; whose end shall be according to their works." (Paul, 2 Cor 11:12-15)
"When a man goes about prophesying, and commands men to obey his teachings, he must either be a true or false prophet. 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." (Joseph Smith, HC 6:17, p. 364)
"...to that being who beguiled our first parents, who transformeth himself nigh unto an angel of light..." (2 Nephi 9:9)
"And with all deceivableness of unrighteousness in them that perish; because they received not the love of the truth, that they might be saved. And for this cause God shall send them strong delusion, that they should believe a lie." (2 Thessalonians 2:10-11)
"There have also been ministering angels in the Church which were of Satan appearing as an angel of light. A sister in the state of New York had a vision, who said it was told her that if she would go to a certain place in the woods, an angel would appear to her. She went at the appointed time, and saw a glorious personage descending, arrayed in white, with sandy colored hair; he commenced and told her to fear God, and said that her husband was called to do great things, but that he must not go more than one hundred miles from home, or he would not return; whereas God had called him to go to the ends of the earth, and he has since been more than one thousand miles from home, and is yet alive. Many true things were spoken by this personage, and many things that were false. How, it may be asked, was this known to be a bad angel? By the color of his hair; that is one of the signs that he can be known by, and by his contradicting a former revelation." (Joseph Smith, HC v. 4, ch. 33)
"The world always mistook false prophets for true ones, and those that were sent of God, they considered to be false prophets, and hence they killed, stoned, punished and imprisoned the true prophets, and these had to hide themselves 'in deserts and dens, and caves of the earth,' and though the most honorable men of the earth, they banished them from their society as vagabonds, whilst they cherished, honored and supported knaves, vagabonds, hypocrites, impostors, and the basest of men." (HC v. 4, ch. 33, p. 573)
"I am going on in my progress for eternal life....Oh! I beseech you to go forward, go forward and make your calling and your election sure; and if any man preach any other Gospel than that which I have preached, he shall be cursed; and some of you who now hear me shall see it, and know that I testify the truth concerning them." (Joseph Smith, HC 6:17)