We know that missionary work is part of God's plan for His children and has been from the time of Adam. Moses 5:58 tells us that Jesus Christ revealed the gospel plan to Adam: "And thus the Gospel began to be preached, from the beginning." Later generations of Adam's righteous descendants were sent to preach the gospel, and they "called upon all men everywhere, to repent; and faith was taught unto the children of men" (Moses 6:23).
Each prophet in his own day was a missionary and was obedient to the commandment to preach the gospel message. The Lord has called missionaries to preach the eternal principles of the gospel to his children whenever priesthood authority has been on earth. We know that the Lord's Church has always been a missionary church because Jesus Christ ordained Apostles and Seventies with priesthood power and authority and sent them out to preach the gospel. Most of the preaching done by the early Apostles was to the Jews (see Matthew 10:5-6). Then, following His resurrection, Jesus Christ told Peter that the gospel was for the whole world and sent His Apostles to preach the gospel to the Gentiles. His commandment to the Apostles is the same as in our day, "Go ye into all the world, and preach the gospel to every creature" (Mark 16:15).
One of the greatest missionaries in all time was the Apostle Paul who was sent to the Gentiles and spent the rest of his life preaching the gospel to them. He continued to preach the gospel to them even though he was at different times whipped, stoned, and imprisoned (see Acts 23:10-12; 26).
When the Lord restored His Church through the Prophet Joseph Smith, missionary work started anew. The chief responsibility for preaching the gospel and making sure it is preached in all the world lies with the Apostles and Seventies. Samuel Harrison Smith, the Prophet's brother, began the first missionary journey for the Church in June 1830 in answer to the Lord's commandment to Joseph Smith: "Proclaim my gospel from land to land, and from city to city…. Bear testimony in every place, unto every people" (Doctrine and Covenants 66:5, 7).
Today, over one million missionaries have been called, set apart, and sent forth to preach the message that Jesus Christ is the Son of God and our Savior. Missionaries also testify that God has restored His gospel once again to earth through His chosen prophet. All missionaries have the responsibility to preach the gospel to all people, to baptize them, and to teach them the commandments of God (see Matthew 28:19-20). These missionaries go to all parts of the world at their own expense for the specific reason to preach the gospel message.
The Lord requires His Church to take His restored gospel to every nation and people (see Doctrine and Covenants 133:37). We know that the Lord does not give any commandment without also preparing a way for that commandment to be accomplished (see 1 Nephi 3:7). Under the Lord's direction and through His preparations, missionaries have been able to teach the gospel in nations that were once closed to us. We know that the Lord will open the doors of other nations to missionary work as we continue to exercise faith and pray for those doors to open.
There are many other ways in which the Lord is helping missionaries to spread the gospel. According to Elder Russell M. Nelson of the Quorum of Twelve Apostles, God is "inspiring the minds of great people to create inventions that further the work of the Lord in ways this world has never known" (see "Computerized Scriptures Now Available," Ensign, April 1988, 73). The gospel message is spread to millions of people through newspapers, magazines, television, radio, satellites, computers, the Internet, and related technology. Those who currently enjoy the blessings of the fulness of the gospel of Christ are urged to use these inventions to obey the Lord's commandment: "For, verily, the sound must go forth from this place into all the world, and unto the uttermost part of the earth - the gospel must be preached unto every [person]" (Doctrine and Covenants 58:64).
President Ezra Taft Benson explained, "This is our first interest as a Church - to save and exalt the souls of the children of men" (Ensign, May 1974, 104). The importance of missionary work can be better seen with the understanding that all the people of the world must be given an opportunity to hear and accept the gospel. This is so important because every person must have an opportunity to learn the truth, turn to God, and receive forgiveness from their sins.
There are many good people on earth, our brothers and sisters, who are blinded by the false teachings of men and "are only kept from the truth because they know not where to find it" (Doctrine and Covenants 123:12). It is through missionary work that all these wonderful people can receive the truth. It is through missionary work that the world is being prepared for the Second Coming of the Savior (see Doctrine and Covenants 34:6; see also Doctrine and Covenants 43:28).
Every member of The Church of Jesus Christ of Latter-day Saints is a missionary even if we are not formally called and set apart. We are each charged with the responsibility to teach the gospel by word and deed to all of the children of Heaven Father. The Lord said, "It becometh every man who hath been warned to warn his neighbor" (Doctrine and Covenants 88:81).
The Book of Mormon has a story showing how the sons of King Mosiah willingly accepted their responsibility to teach the gospel. Following their conversion to the Church, their hearts were filled with compassion for other people. Their love for others included their enemies and led to their request to preach the gospel to the Lamanites because "they could not bear that any human soul should perish; yea even the very thoughts that any soul should endure endless torment did cause them to quake and tremble" (Mosiah 28:3). This kind of love and compassion for others comes to us as the gospel fills our lives with joy and leads to a desire to share the message of the gospel with everyone who will listen.
Missionaries receive great blessings: "Ye are blessed, for the testimony which ye have borne is recorded in heaven for the angels to look upon; and they rejoice over you" (Doctrine and Covenants 62:3). Working for the salvation of other people brings forgiveness for our own sins and salvation for our own souls (see Doctrine and Covenants 4:4; 31:5; 84:61). The Lord said, "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!" (Doctrine and Covenants 18:15-16).
Teaching the gospel to others and then seeing them receive blessings from obedience to those teachings brings the sweetest joy I know.