Missionary work is the responsibility of every church member. Why?
"So that others may receive the saving ordinances in The Church of Jesus Christ by inviting them to come unto Christ. The message of the restoration is this invitation to know why the gospel of Jesus Christ and his true church have been restored by a prophet in modern times."
Members can extend such an invitation by declaring that God lives, that he loves us and that he is a God of revelation. That can be known through revelation and the witness of the prophets.
Adam, the first prophet, obtained a sure witness that Jesus was the redeemer of the world. His major responsibility was to preserve the truth of the gospel as well as teach it as it was given to him. Satan, on the other hand, induced people into confusion and apostasy.
The Old Testament is a religious history of continuous revelations through prophets like Noah, Abraham and Moses.
The New Testament confirms the teachings and prophecies of Old Testament prophets. It is an account of the life and ministry of Jesus Christ.
"The message of the New Testament was clear: There was one fold; one faith; one gospel; one priesthood; one church in order to "be in one, the children of Christ.' "
But rejection of Christ's gospel brought about another apostasy. And once again, it was followed by a restoration. Joseph Smith received full divine doctrinal knowledge and the authority and keys of the priesthood.