The phrase “eat, drink, and be merry, for tomorrow we die” is a familiar description of the lack of care one might have for the future. In Nephi’s usage, it follows the description of the believers in the accomplished God, and his usage absolutely hinges on that concept. In the context of a population believing that God has already accomplished his major task, there is nothing to look forward to. There is no future redemption, for the redemption has already occurred. They need not worry about a future judgement, for the “judgement” is already in the past - the Savior will accomplish the atonement, and living under the atonement, they are already “saved.” Once again, Nephi presents this argument from the positive view, but he does so precisely because he sees it as so obviously incorrect that placing it in its most positive light only highlights the incongruity of the position. For Nephi, this is a concept so clearly incorrect that it nearly refutes itself (though Nephi will continue to deride the mindset that produces the idea).
In this verse, Nephi once again echoes Isaiah. However, the very specific mention of the troika of words “eat, drink, merry” is not unusual in the Old Testament. In perhaps one of the more interesting usages, the trio of terms is used very positively:
14 There is a vanity which is done upon the earth; that there be just men, unto whom it happeneth according to the work of the wicked; again, there be wicked men, to whom it happeneth according to the work of the righteous: I said that this also is vanity.
15 Then I commended mirth, because a man hath no better thing under the sun, than to eat, and to drink, and to be merry: for that shall abide with him of his labour the days of his life, which God giveth him under the sun.
The usage of the terms in Ecclesiastes describes the precise situation - that of concentrating on the day at hand rather than the future. In Ecclesiastes, however, the reason given for focusing on the current day comes in the contrast with the inequities established in verse 14. Because the world is full of things that are not fair, one must take joy where one can. Thus the message of Ecclesiastes is wholly focused on the events of this mortal life, with no discussion of future redemption. It is in that temporally focused context that the “eat, drink, merry” set becomes an affirmation of life - a way to live joyfully in spite of the surrounding inequities.
Isaiah’s usage of the concept is different, however, precisely because there is a moral factor entering in to the equation. Where Ecclesiastes is focusing on the way one’s daily life can be lived in happiness, Isaiah is using the terms as a contrast to a righteous way of living:
11 Ye made also a ditch between the two walls for the water of the old pool: but ye have not looked unto the maker thereof, neither had respect unto him that fashioned it long ago.
12 And in that day did the Lord GOD of hosts call to weeping, and to mourning, and to baldness, and to girding with sackcloth:
13 And behold joy and gladness, slaying oxen, and killing sheep, eating flesh, and drinking wine: let us eat and drink; for to morrow we shall die.
14 And it was revealed in mine ears by the LORD of hosts, Surely this iniquity shall not be purged from you till ye die, saith the Lord GOD of hosts.
Verse 11 of Isaiah 22 sets up the context. Jerusalem is preparing itself against siege from an enemy. In doing so, they take care with the worldly aspects, and in particular their water supply - a pool of water. The Lord notes that while they care for the pool, they neglect the one who created the pool. The Lord has set up a condition of a wayward Judah, paying attention to the things of the world while ignoring the things of the Spirit.
The contrast is heightened in verses 12-13. The Lord commands weeping and mourning - or repentance and humility, and the people instead are eating and drinking. The eating and drinking is still in the context used by Ecclesiastes, that of daily living. Rather than repent, Jerusalem is continuing with their daily way of life. They continue as if there were nothing different. They do not see the error of their ways and turn to the Lord.
The “tomorrow we die” phrase highlights the temporal vision of the people. Notice that the “merry” of the standard trio of terms is missing. Isaiah is not contrasting a “merry” people with his desired repentant people, but rather their lack of focus on the things of the Spirit - those things which pertain to more than “tomorrow.”
Nephi is following Isaiah’s usage, though with the addition of the “merry.” The concept is still the quotidian focus rather eschatological focus.