Blind

And after these plain and precious things were taken away it goeth forth unto all the nations of the Gentiles; and after it goeth forth unto all the nations of the Gentiles, yea, even across the many waters which thou hast seen with the Gentiles which have gone forth out of captivity, thou seest—because of the many plain and precious things which have been taken out of the book, which were plain unto the understanding of the children of men, according to the plainness which is in the Lamb of God—because of these things which are taken away out of the gospel of the Lamb, an exceedingly great many do stumble, yea, insomuch that Satan hath great power over them. - 1 Nephi 13:29

The apostasy didn't happen over night. It was a process. One of the key elements, after killing the 12 Apostles, was to slightly alter or remove certain things from the doctrine of Christ. It is no wonder that people become confused with some of the teaching that comes from the Bible. Parts have been removed and others altered. The blind have lead the blind and now both have fallen into the ditch (Matthew 15:14).

I'm so glad we have Prophets, Apostles, personal revelation, and the Book of Mormon, in addition to the Bible. It is harder to be confused when you have multiple sources for learning he gospel of Jesus Christ. (See also Isaiah 29:18, Isaiah 42:16, Amos 8:11-14, Matthew 7:21)