Jeffrey R. Holland
"The potential for confusion here comes with the realization that many (and perhaps all) of the major prophets living prior to the brother of Jared had seen God. How, then, do we account for the Lord’s declaration? …before the time of the brother of Jared, the Lord did appear to Adam and ‘the residue of his posterity who were righteous’ in the valley of Adam-ondi-Ahman three years before Adam’s death (DC 107:53). And we are left with Enoch, who said explicitly, ’I saw the Lord; and he stood before my face, and he talked with me, even as a man talketh one with another, face to face.’ (Moses 7:4) We assume that other prophets between the Fall and the Tower of Babel saw God in a similar manner, including Noah, who ‘found grace in the eyes of the Lord’ and ’walked with God,’ (Gen 6:8-9) the same scriptural phrase used to describe Enoch’s relationship with the Lord. (Gen 5:24)
"This issue has been much discussed by Latter-day Saint writers, and there are several possible explanations, any one—or all—of which may cast light upon the larger truth of this passage. Nevertheless, without additional revelation or commentary on the matter, any conjecture is only that and as such is inadequate and incomplete.
"One possibility is…that the reference to ‘man’ is the key to this passage, suggesting that the Lord had never revealed himself to the unsanctified, to the nonbeliever, to temporal, earthy, natural man. The implication is that only those who have put off the natural man, only those who are untainted by the world—in short, the sanctified (such as Adam, Enoch, and now the brother of Jared)—are entitled to this privilege.
"Some believe that the Lord meant he had never before revealed himself to man in that degree or to that extent. This theory suggests that divine appearances to earlier prophets had not been with the same ’fulness,‘ that never before had the veil been lifted to give such a complete revelation of Christ’s nature and being…
"A final explanation—and in terms of the brother of Jared’s faith the most persuasive one—is that Christ was saying to the brother of Jared, ’Never have I showed myself unto man in this manner, without my volition, driven solely by the faith of the beholder.’ As a rule, prophets are invited into the presence of the Lord, are bidden to enter his presence by him and only with his sanction. The brother of Jared, on the other hand, seems to have thrust himself through the veil, not as an unwelcome guest but perhaps technically as an uninvited one. Said Jehovah, ’Never has man come before me with such exceeding faith as thou hast; for were it not so ye could not have seen my finger… . Never has man believed in me as thou hast.’ (v. 9,15) Obviously the Lord himself was linking unprecedented faith with this unprecedented vision. If the vision itself was not unique, then it had to be the faith and how the vision was obtained that was so unparalleled. The only way that faith could be so remarkable was its ability to take the prophet, uninvited, where others had been able to go only with God’s bidding.
“That appears to be Moroni’s understanding of the circumstance when he later wrote, ’Because of the knowledge [which came as a result of faith] of this man he could not be kept from beholding within the veil… . Wherefore, having this perfect knowledge of God, he could not be kept from within the veil; therefore he saw Jesus.’ (v. 19)” (Christ And The New Covenant, p. 21-23)
Joseph Fielding Smith
“I have always considered Ether 3:15 to mean that the Savior stood before the Brother of Jared plainly, distinctly, and showed him his whole body and explained to him that he was a spirit. In his appearance to Adam and Enoch, he had not made himself manifest in such a familiar way. His appearances to earlier prophets had not been with that same fulness.” (Doctrines of Salvation, 1:37)