The angel clearly teaches Nephi that the book that "proceedeth out of the mouth of a Jew" (the Bible) contained the plainness of the gospel until after it came into the hands of a great and abominable church. In fact, the angel seems to indicate that this church is called an abominable church because it has taken away "from the gospel of the Lamb many parts which are plain and most precious." (1 Nephi 13:26.)
Thus, apparently the first part of our present Bible (the Old Testament) is incomplete because of deletions and changes made by some of the wicked "pastors" of the house of Israel (see 1 Nephi 21:1), and the second part of our Bible (the New Testament) has been altered and changed because of the influence of "a great and abominable church." One purpose for the coming forth of the Book of Mormon is to restore these plain and precious truths that have been deleted or altered in the "stick of Judah."