What are the words established by the mouth of Jesus which are made known in the Book of Mormon and in the New Testament?1 Nephi 13:40-41
And the angel spake unto me, saying: These last records, which thou hast seen among the Gentiles, shall establish the truth of the first, which are of the twelve apostles of the Lamb, and shall make known the plain and precious things which have been taken away from them; and shall make known to all kindreds, tongues, and people, that the Lamb of God is the Eternal Father, and the Savior of the world; and that all men must come unto him, or they cannot be saved.
And they must come according to the words which shall be established by the mouth of the Lamb; and the words of the Lamb shall be made known in the records of thy seed, as well as in the records of the twelve apostles of the Lamb; wherefore they both shall be established in one; for there is one God and one Shepherd over all the earth.
Those are the words by which all men, us included, must come unto Christ or we cannot be saved.
Those words, spoken by the mouth of Jesus, are found in Luke 6:20-49, Matthew chapters 5 through 7, and 3 Nephi chapters 12 through 14.
We know those are the words which we must observe to do to come to Jesus because of what Jesus said by his own mouth as recorded in the Book of Mormon.
A man would be far better served to observe to strictly and literally keep and to teach the commandments of Jesus Christ found in 3 Nephi chapters 12 through 14 than he would to spend his life establishing the ancient provenance of the Book of Mormon without observing to do nor teach them.3 Nephi 14
24 Therefore, whoso heareth these sayings of mine and doeth them, I will liken him unto a wise man, who built his house upon a rock—
25 And the rain descended, and the floods came, and the winds blew, and beat upon that house; and it fell not, for it was founded upon a rock.
26 And every one that heareth these sayings of mine and doeth them not shall be likened unto a foolish man, who built his house upon the sand—
27 And the rain descended, and the floods came, and the winds blew, and beat upon that house; and it fell, and great was the fall of it.
JST Matthew 5
Matt 5:21 Whosoever, therefore, shall break one of these least commandments and shall teach men so to do, he shall in no wise be saved in the kingdom of heaven; but whosoever shall do and teach these commandments of the law until it be fulfilled, the same shall be called great and shall be saved in the kingdom of heaven.
Matt 5:22 For I say unto you, except your righteousness shall exceed that of the scribes and Pharisees, ye shall in no case enter into the kingdom of heaven.