Question: Did Jacob share with the Nephites the “doctrine” or “doctrines” of the kingdom?
Answer: In the Book of Mormon, the word doctrine appears twenty-four times, always with a singular meaning. The doctrine of Christ is faith, repentance, baptism, and the gift of the Holy Ghost.
Jacob shared with the people of Nephi the doctrine of the kingdom. After Jacob teaches them about following Christ and about repentance and baptism and the Holy Ghost, he states, “And now, behold, my beloved brethren, this is the way; and there is none other way nor name given under heaven whereby man can be saved in the kingdom of God. And now, behold, this is the doctrine of Christ, and the only and true doctrine of the Father, and of the Son, and of the Holy Ghost...Amen.” (2 Nephi 31:21)
When the word doctrine appears in the plural, it has reference to foolish or false teachings that deny Jesus as the Christ.
“Yea, and there shall be many which shall teach after this manner, false and vain and foolish doctrines, and shall be puffed up in their hearts...” (2 Nephi 28:9)
Source: 400 Questions and Answers About the Book of Mormon by Susan Easton Black, p. 100.