It is perhaps safe to say that the Sermon on the Mount is the most quoted and the least understood of all the teachings of Jesus. The translation of Matthew’s account of this sermon in our present New Testament (Matthew 5–7) has caused many people to raise questions concerning the authenticity of the sermon. They ask such questions as the following:
When the resurrected Jesus Christ appeared to the Nephites, he gave them the same sermon. In fact, he specifically told the Nephites, ‘Behold, ye have heard the things which I taught before I ascended to my Father.’ (3 Nephi 15:1.) However, the account of this sermon in the Book of Mormon is much more complete and makes much more sense than the New Testament account.
For example, in the Book of Mormon the Savior prefaced his sermon by indicating the teachings that were to follow applied only to those who would ‘come down into the depths of humility and be baptized … [and] be visited with fire and with the Holy Ghost, and … receive a remission of their sins.’ (3 Nephi 12:2.) Then he related these prerequisite conditions to each of the Beatitudes that followed: ‘Yea, blessed are the poor in spirit who come unto me, for theirs is the kingdom of heaven. And again, blessed are all they that mourn, for they shall be comforted.’ (3 Nephi 12:3–4 Italics added.)
This more complete version of the sermon changes the entire emphasis of the Beatitudes. Here the Savior is not saying ‘you are more blessed if you mourn than if you do not mourn, ’ but he is saying ‘If you are called upon to mourn, then you are blessed if you come unto me, are baptized, receive the Holy Ghost, etc.’ Thus, if you do truly hunger and thirst after righteousness, you shall be filled ‘with the Holy Ghost.’ (3 Nephi 12:6.)
It is also of interest to note that each of the Beatitudes in the Book of Mormon begins with the coordinating conjunction ‘and, ’ which helps to relate them back to the introductory statement.
(Daniel H. Ludlow, A Companion to Your Study of the Book of Mormon, p. 263-64)