Our greatest and most important duty is to preach the gospel.
What is missionary work?
The Lord has declared that missionary work is the responsibility of all who follow Him (see Matthew 28:19–20; D&C 88:81). Members of the Lord's Church can, by the goodness of their lives and the strength of their testimonies, share the gospel message and help prepare family members, friends, and other acquaintances who are not Latter-day Saints to be taught by the full-time missionaries.
When we experience the blessings of gospel living, we naturally want to share those blessings with others. The Lord spoke of the joy that comes into our lives when we share His gospel:
“If it so be that you should labor all your days in crying repentance unto this people, and bring, save it be one soul unto me, how great shall be your joy with him in the kingdom of my Father!
“And now, if your joy will be great with one soul that you have brought unto me into the kingdom of my Father, how great will be your joy if you should bring many souls unto me!” (D&C 18:15-16).
After His Resurrection, the Lord commanded His disciples to ”go . . . and teach all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Ghost“ (Matthew 28:19). In fulfillment of this command, able young men in the Church have a duty to prepare spiritually, physically, and emotionally to serve as full-time missionaries. Single women and mature couples also have the opportunity to serve full-time missions.
Missionary work also includes helping and supporting those who join the Church. As instructed by President Gordon B. Hinckley, each new member of the Church needs three things: a friend, a responsibility, and nurturing with “the good word of God” (Moroni 6:4; see also “Converts and Young Men,” Ensign, May 1997, 47–50).
See True to the Faith (2004), 104-6