The Book of Mormon gives an account of the resurrected Jesus appearing to a people here on the American continent (shortly after his ascension in the Old World). During that appearance, Jesus repeated a sermon that he gave during his mortal life, the sermon which has come to be known as the "Sermon on the Mount", which comprises chapters 5, 6 & 7 of Matthew. Noticing this, some Book of Mormon critics have accused Joseph Smith of merely (and lazily) slapped in portions of the Bible into the Book of Mormon. However, what they may not have noticed is that there are some differences between the two sermons, some small, some quite large and important. Here below is a chart which compares the two sermons. The differences will be notated in RED, and the letters "NA" will be used to indicate when a whole verse is missing is in one book but not in the other.
First, a few points to make as we consider the two sermons:
SAME MESSAGE: The Sermon on the Mount is the longest sermon, the first sermon, and the most well known sermon that Jesus ever preached during his mortal ministry in the Old World (one which he taught, at least in part, multiple times during his ministry). That sermon clearly has great significance and benefit for mankind. The fact that this same important sermon was also given to this other branch of the House of Israel confirms that the Bible and the Book of Mormon both truly teach the same one gospel message and that both peoples (and all the world) have the same one Shepherd, Jesus Christ,the Son of God.
NEW INSIGHT: Many of the changes on the Book of Mormon give wonderful insight and clarification. For example, instead of simply saying, "Blessed are the poor in spirit", it reads, "Blessed are the poor in spirit that come unto me", which clarifies that it is not merely being down in spirit that qualifies one for the blessing (which, in this case, is the "Kingdom of Heaven"), but it is the "coming unto Christ" that so qualifies us. Another insight can be found in the fact instead of inviting the people to be perfect "even as your Father which is in heaven is perfect", Jesus says, "even as I, or your Father who is in heaven is perfect", pointing out that Jesus too had reached a state of full perfection (now as a resurrected and glorified being that had fulfilled his mission) which could not be said of him during his mortal life.
MATCHES CONTEXT: Various changes perfectly fit the new context of Book of Mormon sermon. For example, the Book of Mormon sermon excludes the mention of "Jerusalem" and "publicans" since those terms clearly do the Book of Mormon people.
THE GOSPEL & AUTHORITY: It is also worth noting that the Book of Mormon sermon starts off with TWO NEW VERSES which teach of the blessedness of those who are "humble" themselves, "believe in [Christ]", are "baptized" and receive the "Holy Ghost" for "the remission of sins" and who "give heed" to the Lord's chosen servants to whom he gave priesthood "power" to "minister" to the church and to perform certain commanded ordinances. What a beautiful and clear summary of the saving gospel of Jesus Christ.