Compare verse 24
"Thou hast beheld that the book proceeded forth from the mouth of a Jew; and when it proceeded forth from the mouth of a Jew it contained the fulness of the gospel of the Lord"
with verse 28
"Wherefore, thou seest that after the book hath gone forth through the hands of the great and abominable church, that there are many plain and precious things taken away from the book"
How can a book come from the mouth of a Jew? This is clearly an allusion to oral traditions which is the origin of the written bible. But the hands of the great and abominable church refers to writing and apparently changing the written words before it became the bible as we know it.