(Isa. 29:4; refer in this text to 2 Ne. 18:19.)
Elder LeGrand Richards called our attention to a minister of another religion who said that the Book of Mormon read with the same sweet feeling as the New Testament (see CR, Apr. 1976, 124). In other words, the message of the Book of Mormon sounds familiar to those who have received the message of the Bible, the gospel, or the Spirit of God. On the other hand, “familiar spirits” in Old Testament times referred to a form of spiritualism wherein spirits from the dead were called back out of the ground through witchcraft or enchantment (see 1 Sam. 28; Isa. 8:19–20). Some have used this meaning to disclaim the Book of Mormon. However, a careful reading of Isaiah shows that the words of the record would be “as” a familiar spirit. Just as the spirits came “out of the ground,” so would the record of a people who had been destroyed. Nephi in 2 Nephi 26:16 shows that the power of God would be given to these people to “whisper … even as it were out of the ground.”
(Monte S. Nyman, Great Are the Words of Isaiah [Salt Lake City: Bookcraft, 1980], 107–108.)
Some biblical scholars have maintained that witchcraft is being referred to in that portion of Isaiah 29:4 which says that the voice shall be “as one that hath a familiar spirit.” These scholars evidently arrived at this interpretation because of similar wording in other parts of the Bible. For example, in Leviticus we read: “Regard not them that have familiar spirits, neither seek after wizards, to be defiled by them” (Leviticus 19:31), and “A man also or woman that hath a familiar spirit, or that is a wizard, shall surely be put to death” (Leviticus 20:27). (For further biblical references indicating that the term “familiar spirits”might sometimes refer to witches, see 1 Sam. 28:7; 2 Kgs. 21:6; 1 Chron. 10:13; Isa. 8:19 and 19:3.)
However, a careful reading of this scripture, particularly when read together with Nephi’s explanation, would indicate that the term it “hath a familiar spirit” means that this record (the Book of Mormon) would speak with a “familiar voice” to those who already have the Bible. In other words, Nephi is evidently saying here that the doctrinal teachings of the Book of Mormon would seem familiar to people who had already read and accepted the Bible.”
(Daniel H. Ludlow, A Companion To Your Study of The Book of Mormon [Salt Lake City: Deseret Book Co., 1976], 138–139.)