Many a street preacher has gone to and fro around the United States and the world preaching what he termed to be the gospel of Christ. However, when one looks at those preachers in light of the Restored Gospel, one finds that said preachers are in error, because they have not been called of God. Indeed, Paul said it best when he wrote, "And no man taketh this honor [the Priesthood or Authority of God to preach the Gospel] unto himself, but he that is called of God, as was Aaron. (Hebrews 5:4)" How was Aaron called? Why, through a prophet of the Lord, even his brother Moses (Ex. 28:1).
It follows then that no man nor woman is authorized to preach the Gospel of Christ unless called by revelation through a prophet. Elder Orson Pratt said, "No person was ever authorized to act in the name of the Lord, unless called by new revelation. (The Essential Orson Pratt, p. 55)" In order for anyone, then, to be called to preach the Gospel in any capacity, it must be done through living prophets which said prophets are only found within The Church of Jesus Christ of Latter-day Saints.
An example from former days is the calling of Matthias to be ordained as one of the Twelve Apostles. This calling was directed by Peter (the President of the Church) in a speech given to the rest of the Eleven, as seen in Acts 1:15-26.
An example in these latter days of one being called to preach the Gospel of Christ by the Lord through a prophet of God is my own missionary call. It reads:
Dear Elder Herlin:
You are hereby called to serve as a missionary of The Church of Jesus Christ of Latter-day Saints. You are assigned to labor in the California Carlsbad Mission. . . .
[Signed] Gordon B. Hinckley
I hope this essay is sufficient to explain the need of one to be called of God through a prophet, in order to preach the Gospel. I know that The Church of Jesus Christ of Latter-day Saints is true, and the only Church Authorized by God to appropriately call men and women to preach the Gospel of Salvation. I know that both the Bible and Book of Mormon teach of the necessity of being called of God. In the name of Jesus Christ, amen.