Brigham Young said something that I think is important here: "Learning a, b, c, d, does not hinder me from learning e, f, g" (Discourses of Brigham Young, p. 245). In other words (and to adapt his quote to this essay), just because someone happens to know a and b does not mean they cannot also know c, d, and e. Believing in the Bible does not preclude believing in the Book of Mormon. Or, having the Book of Mormon does not lessen the importance of the Bible. In reality it increases the importance of the Bible because it stands as another witness to the veracity of the Bible. We have that additional testimony that the Bible is true! "In the mouth of two or three witnesses shall every word be established" (2 Cor. 13:1; see also Deut. 17:6, Matt. 18:16, D&C 6:28). God establishes the truth of His words through many witnesses. However, many believe that the Bible is enough. The Lord cautioned those who state that the Bible is all they need.
"And because my words shall hiss forth—many of the Gentiles shall say: A Bible! A Bible! We have got a Bible, and there cannot be any more Bible.... Thou fool, that shall say: A Bible, we have got a Bible, and we need no more Bible. Have ye obtained a Bible save it were by the Jews? Know ye not that there are more nations than one?...Wherefore murmur ye, because that ye shall receive more of my word? Know ye not that the testimony of two nations is a witness unto you that I am God, that I remember one nation like unto another?Those are strong words but sometimes the truth is hard to bear. However, they are also very loving words; they show God's care and concern for all His children. He speaks to many nations so they may know His words of great joy and love. He wants all His children to hear His words. I think all would be glad to know that God speaks to us today (except for those who find it more convenient to believe God is dead, God doesn't exist, or God at least only spoke to a bunch of old dead guys)! His words and works do not cease. His work and glory is to provide eternal life to as many as will accept it (see Moses 1:39). I testify that the Book of Mormon contains the words of Christ. It builds, strengthens, and fortifies the Bible.
Wherefore, I speak the same words unto one nation like unto another. And when the two nations shall run together the testimony of the two nations shall run together also. And I do this that I may prove unto many that I am the same yesterday, today, and forever; and that I speak forth my words according to mine own pleasure. And because that I have spoken one word ye need not suppose that I cannot speak another; for my work is not yet finished; neither shall it be until the end of man, neither from that time henceforth and forever. Wherefore, because that ye have a Bible ye need not suppose that it contains all my words; neither need ye suppose that I have not caused more to be written....
For behold, I shall speak unto the Jews and they shall write it; and I shall also speak unto the Nephites and they shall write it; and I shall also speak unto the other tribes of the house of Israel, which I have led away, and they shall write it; and I shall also speak unto all nations of the earth and they shall write it. And it shall come to pass that the Jews shall have the words of the Nephites, and the Nephites shall have the words of the Jews; and the Nephites and the Jews shall have the words of the lost tribes of Israel; and the lost tribes of Israel shall have the words of the Nephites and the Jews" (2 Nephi 29:3,6-13; emphasis added).