In Nephi's own words, the guilty taketh the truth to be hard, for it cutteth them to the very center (1 Nephi 16:2). Speaking to Joseph Smith of the Book of Mormon, the Lord said, Satan will harden the hearts of the people to stir them up to anger against you, that they will not believe my words (DC 10:32). Therefore, if one encounters a strident opponent to the Book of Mormon and the individual is angry with the message of the Book of Mormon, you can know for a surety that he is of the spirit of the devil.