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: Why did the Savior teach that people were better off being poor in spirit than not poor in spirit, or that they were more blessed mourning than not mourning? When he said "Blessed are they which do hunger and thirst after righteousness: for they shall be filled," with what are they to be filled: hunger, thirst, or righteousness? Also, why did he counsel the people to "take no thought for your life, what ye shall eat, or what ye shall drink; nor yet for your body, what ye shall put on"? What if all people literally followed this admonition? Who would plant and harvest the crops, feed the children, manufacture the clothes, etc. It is evident there is something wrong with the translation of the Sermon on the Mount in our present New Testament.
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.