The imagery of the a never-ending fire was available to Benjamin from the small plates of Nephi (2 Ne. 9:16, Jacob 6:10). The closest match to Benjamin’s statement is Jacob 6:10; “And according to the power of justice, for justice cannot be denied, ye must go away into that lake of fire and brimstone, whose flames are unquenchable, and whose smoke ascendeth up forever and ever, which lake of fire and brimstone is endless torment.” The clearly common phrases between Benjamin and Jacob “never-ending torment/endless torment” and the “unquenchable” fire. While we cannot be absolutely certain that Benjamin takes his cue from Jacob, it is quite possible since Benjamin received the small plates directly from Amaleki, and surely would have read them after receiving them.
We start with Benjamin’s conclusion in verse 39 that “mercy hath no claim on that man; therefore his final doom is to endure a never-ending torment.” First, it is very clear that Benjamin is talking about torment, and a final judgement. For the type of person Benjamin is describing, the consequence of his/her action is a “never-ending torment”. As with Jacob, Benjamin likens this torment to being burned by fire. While that is certainly a dreadful image, the real import of Benjamin’s description is not to scare through the fear of the result, but rather to describe the type of person to whom such a torment would apply.
First, Benjamin very specifically limits this type of punishment to one who is both unrepentant, and dies “an enemy to God.” These are two different conditions. One may be unrepentant of many sins, but not necessarily be an “enemy to God.” Verse 37 describes the “enemy to God” as one who is in open rebellion – in other words, one who has known the gospel, and has turned against it. It is this person who has elected to turn against God that becomes a willing enemy to God.
It is important that Benjamin also notes that being an enemy to God is not the sole criteria for assigning a soul to this eternal torment. That “enemy” must also be unrepentant - dying in the state of enmity to God. Thus Benjamin is discussing a grievous sin, but not one from which there is no repentance. Presumably, one’s individual agency would allow for repentance.
The next fascinating aspect is Benjamin’s very clear understanding of the way in which this torment occurs. We might expect that God would assign the torment as the just dessert for one who dies as an enemy to God (thereby remaining an enemy while even closer to God’s presence in the next life). Nevertheless, that is not what Benjamin describes: “the demands of divine justice do awaken his immortal soul to a lively sense of his own guilt, which doth cause him to shrink from the presence of the Lord, and doth fill his breast with guilt, and pain, and anguish, which is like an unquenchable fire, whose flame ascendeth up forever and ever.”
The torment is internal, not external. It is self-imposed, not levied by a vengeful and wrathful God. The pain and anguish are not literal fires, but are rather like and unquenchable fire. Benjamin is placing the blame for the application of the penalty of judgement squarely on the shoulders of the sinner. The torment is not less, the difference is from whence it is assigned. In this, Benjamin presages the Doctrine & Covenants which also has the individual playing a large role in their final judgements in the next life:
"D&C 88:22 For he who is not able to abide the law of a celestial kingdom cannot abide a celestial glory.
23 And he who cannot abide the law of a terrestrial kingdom cannot abide a terrestrial glory.
24 And he who cannot abide the law of a telestial kingdom cannot abide a telestial glory; therefore he is not meet for a kingdom of glory. Therefore he must abide a kingdom which is not a kingdom of glory."
Not only does each person “abide” the kingdom for which they can “abide” the law, but they are also essentially open to all celestial rewards: “D&C 88:31And also they who are quickened by a portion of the telestial glory shall then receive of the same, even a fulness. 32 And they who remain shall also be quickened; nevertheless, they shall return again to their own place, to enjoy that which they are willing to receive, because they were not willing to enjoy that which they might have received.”
As Benjamin points out, the punishment is brought on by one’s own choices, not the least of which is the choice to not repent.