D&C 3:5 says, "Behold, you have been entrusted with these things, but how strict were your commandments; and remember also the promises which were made to you, if you did not transgress them."
This verse contains three important elements that we should carefully consider with respect to the translation of the Book of Mormon and other aspects of the Restoration.
1. What was Joseph entrusted with?
Moroni related the history of the "aborigines of this country" and "said this history was written and deposited not far from that place," meaning the Smith farm near Palmyra.
Moroni told Joseph about the plates, and also "that there were two stones in silver bows—and these stones, fastened to a breastplate, constituted what is called the Urim and Thummim—deposited with the plates; and the possession and use of these stones were what constituted “seers” in ancient or former times; and that God had prepared them for the purpose of translating the book." (Joseph Smith—History 1:35)
Moroni told Joseph he could not show the Nephite relics except under certain conditions. "Again, he told me, that when I got those plates of which he had spoken—for the time that they should be obtained was not yet fulfilled—I should not show them to any person; neither the breastplate with the Urim and Thummim; only to those to whom I should be commanded to show them; if I did I should be destroyed." (Joseph Smith—History 1:42)
Another strict commandment Joseph received was to do the work with an eye single to the glory of God. Oliver explained that, "On the subject of bringing to light the unsealed part of this record, it may be proper to say, that our brother was expressly informed, that it must be done with an eye single to the glory of God; if this consideration did not wholly characterize all his procedings in relation to it, the adversary of truth would overcome him, or at least prevent his making that proficiency in this glorious work which he otherwise would."
When Joseph went to get the plates, he was unable to because he allowed himself to think of the worldly value of the ancient Nephite relics. Moroni explained, "Those who kept the commandments of the Lord on this land, desired this at his hand, and through the prayer of faith obtained the promise, that if their descendants should transgress and fall away, that a record might be kept and in the last days come to their children. These things are sacred, and must be kept so, for the promise of the Lord concerning them must be fulfilled. No man can obtain them if his heart is impure, because the<y> contain that which is sacred; and besides, should they be entrusted in unholy hands the knowledge could not come to the world, because they cannot be interpreted by the learning of this generation; consequently, they would be considered of no worth, only as precious metal. Therefore, remember, that they are to be translated by the gift and power of God."
3. What promises were made to him, if he did not transgress?
In Letter IV, Oliver quoted Moroni's promises to Joseph. "He has therefore chosen you as an instrument in his hand to bring to light that which shall perform his act, his strange act, and bring to pass a marvelous work and a wonder. Wherever the sound shall go it shall cause the ears of men to tingle, and wherever it shall be proclaimed, the pure in heart shall rejoice, while those who draw near to God with their mouths, and honor him with their lips, while their hearts are far from him, will seek its overthrow, and the destruction of those by whose hands it is carried. Therefore, marvle not if your name is made a derission, and had as a by-word among such, if you are the instrument in bringing it, by the gift of God, to the knowledge of the people.”
Moroni also told Joseph that it would be his "privilege, if obedient to the commandments of the Lord, to obtain and translate the same by the means of the Urim and Thummim, which were deposited for that purpose with the record."
In the Wentworth letter, Joseph explained the promises Moroni made to him and how he fulfilled one of them. "I was informed that I was chosen to be an instrument in the hands of God to bring about some of his purposes in this glorious dispensation....
Through the medium of the Urim and Thummim I translated the record by the gift, and power of God."
Despite setbacks (the loss of the 116 pages) and serious obstacles including poverty, the death of his firstborn child, Emma's poor health, and the difficulty of protecting the plates, Joseph eventually accomplished the tasks he was given.