The importance of bearing witness that Joseph Smith is a prophet of God.
Brigham Young used the following story to teach this principle:
"One of our Elders with whom I was acquainted, after he was baptised, got cornered up, and was obliged to preach a sermon. He never had been able to say that he knew Joseph was a Prophet; but he was there in the meeting: the house was crowded with the congregation; the windows and doors full of people, and all around on the green waiting to hear a 'Mormon' preacher.
There were none there but this one man, and he was called upon to preach. He thought he would pray and dismiss the meeting. He never had known that Joseph Smith was a Prophet: that was the lion that lay in his path; and he could not get by him, nor round about him, nor dig under him, nor leap over him; and the lion he must meet: he must say Joseph, for better or worse.
As soon as he got 'Joseph' out, 'is a Prophet' was the next; and from that, his tongue was loosened, and he continued talking until near sundown. The Lord pours out his Spirit upon a man when he testifies that which the Lord gives him to testify of. From that day to this, he has never been at a loss to know that Joseph was a Prophet."
Similarly, Matthew Cowley shared this experience:
"I will never forget the prayers of my father the day that I left. I have never heard a more beautiful blessing in all my life. Then his last words to me at the railroad station, 'My boy, you will go out on that mission; you will study; you will try to prepare your sermons; and sometimes when you are called upon, you will think you are wonderfully prepared, but when you stand up, your mind will go completely blank.' I have had that experience more than once.
"I said, 'What do you do when your mind goes blank?'
"He said, 'You stand up there and with all the fervor of your soul, you bear witness that Joseph Smith was a prophet of the living God, and thoughts will flood into your mind and words to your mouth, to round out those thoughts in a facility of expression that will carry conviction to the heart of everyone who listens.'
And so my mind, being mostly blank during my five years in the mission field, gave me the opportunity to bear testimony to the greatest event in the history of the world since the crucifixion of the Master. Try it sometime, fellows and girls. If you don't have anything else to say, testify that Joseph Smith was the prophet of God, and the whole history of the Church will flood into your mind."
One classic illustration of being true to our testimony that Joseph Smith is a prophet centers on a story that President David O. McKay was fond of telling about a missionary experience of his father, who served in his native Scotland:
"I know that the Lord communicates with his servants. I have not doubted this as a fact since I was a boy and heard the testimony of my father regarding the revelation that came to him of the divinity of the mission of the Prophet Joseph.
I feel impressed to relate that circumstance and add his testimony to the one that I am now giving. He accepted a call to a mission in 1881. When he began preaching in his native land, and bore testimony of the restoration of the gospel of Jesus Christ, he noticed that the people turned away from him. They were bitter in their hearts against anything Mormon, and the name of Joseph Smith seemed to arouse antagonism in their hearts.
One day he concluded that the best way to reach these people would be to preach just the simple principles, the atonement of the Lord Jesus Christ, the first principles of the gospel, and not bear testimony of the restoration.
In a month or so he became oppressed with a gloomy, downcast feeling and he could not enter into the spirit of his work. He did not really know what was the matter; but his mind became obstructed; his spirit became depressed; he was oppressed and hampered; and that feeling of depression continued until it weighed him down with such heaviness that he went to the Lord and said: 'Unless I can get this feeling removed, I shall have to go home. I can't continue my work thus hampered.'
"The discouragement continued for some time after that, when, one morning, before daylight, following a sleepless night, he decided to retire to a cave, near the ocean, where he knew he would be shut off from the world entirely, and there pour out his soul to God and ask why he was oppressed with this feeling, what he had done, and what he could do to throw it off and continue his work.
He started out in the dark towards the cave. He became so eager to get to it that he started to run...Something just seemed to drive him; he had to get relief. He entered the cave or sheltered opening, and said: 'Oh, Father, what can I do to have this feeling removed? I must have it lifted or I cannot continue in this work'; and he heard a voice, as distinct as the tone I am now uttering, say: 'Testify that Joseph Smith is a Prophet of God.'
Remembering then, what he tacitly had decided six weeks or more before, and becoming overwhelmed with the thought, the whole thing came to him in a realization that he was there for a special mission, and he had not given that special mission the attention which it deserved. Then he cried in his heart, 'Lord, it is enough,' and went out from the cave."
Brigham Young illustrated the power associated with the testimony that Joseph Smith is a prophet when he said:
"There is not a man or woman that loves the truth, who has heard the report of the Book of Mormon, but the Spirit of the Almighty has testified to him or her of its truth; neither has any man heard the name of Joseph Smith, but the Spirit has whispered to him-- 'He is a true Prophet.'"