Fear of the unknown
Death is frightening to many people [citation needed] - it represents an unknown, a departure from that which is familiar - these are attributes that make people apprehensive. It is unsurprising that people often look upon death with negative emotion.
For David, death meant leaving behind a remarkably luxurious & powerful mortal existence. As a warrior, he had seen a lot of painful death (e.g. 1 Sam. 18:7), and understandably prayed that this would not be his fate in battle.
For Job, death was the thing that took his family members away (see Job 1:18-19); the reason they were not with him at the moment.
Separation, loss, and the unknown can make death scary even for people who believe in an afterlife.
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All 3 believed there was hope
And yet, this isn't the end of the story. All 3 of the authors referenced in the OP express--in one form or another--a testimony of the resurrection. They're willing to display their human emotion, their fears, their questions--but they don't stop there. Job in particular asks many longing questions that are only answered later in his poetry.
From Job 19:
25 For I know that my redeemer liveth, and that he shall stand at the
latter day upon the earth:
26 And though after my skin worms destroy this body, yet in my flesh
shall I see God
This answers questions Job asked earlier in the story. Recovering from trauma is a process, not an event. Job shares much of that process.
From David, in Psalm 16:
10 For you will not leave my soul in Sheol, you will not allow your
holy one to experience corruption.
A New Testament interpretation of this passage is that David will not be left in Sheol because he will be resurrected--and he will be resurrected because the Holy One will not die and decay (He will die and rise again).
From Isaiah 26:
19 Thy dead men shall live, together with my dead body shall they
arise. Awake and sing, ye that dwell in dust: for thy dew is as the
dew of herbs, and the earth shall cast out the dead.
The songs referenced here sound less like a funeral dirge and more like a celebration.
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Long period of bondage
In this section I will specifically offer a Latter-day Saint perspective.
In his great vision of the spirit world, Joseph F. Smith observed:
And so it was made known among the dead, both small and great, the
unrighteous as well as the faithful, that redemption had been wrought
through the sacrifice of the Son of God upon the cross (Doctrine &
Covenants 138:35)
Up until this time it was all a promise that had yet to be fulfilled. As Ahmad Corbitt observed:
All hope for returning resurrected and clean to God's holy presence
rested entirely upon one Being keeping His promise (source)
Then, after the Savior's mortal mission had been completed, that hope was being realized. It was a new era for the dead as well.
Continuing from Joseph F. Smith:
All these and many more [many of the righteous dead have just been
named]...mingled in the vast assembly and waited for their
deliverance, For the dead had looked upon the long absence of their
spirits from their bodies as a bondage. (Doctrine & Covenants
138:49-50)
I do not claim a solid understanding of how time works in the afterlife, but apparently waiting the equivalent of several centuries for redemption was challenging (if you don't believe waiting--even somewhere comfortable--can be challenging, just ask my children).
Back to Joseph F. Smith once more:
Thus was it made known that our Redeemer spent his time during his
sojourn in the world of spirits, instructing and preparing the
faithful spirits of the prophets who had testified of him in the
flesh; That they might carry the message of redemption unto all the
dead... (Doctrine & Covenants 138:36-37, see also 1 Peter 3:18-20,
4:6)
A new chapter in the spirit world had opened as well--the impassible gulf could be bridged, and messengers could be sent to proclaim the good news to those who had not received it (or had only a partial understanding). I propose that Isaiah himself rejoiced at the prospect that this would eventually happen:
The people that walked in darkness have seen a great light: they that
dwell in the land of the shadow of death, upon them hath the light
shined. (Isaiah 9:2)
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Specific passages
Psalm 115:17
This Psalm contrasts the works of God in heaven with those of men on earth (see verses 3-4).
This theme is emphasized again at the end of the Psalm, as the Psalmist speaks of the blessings God provides to people on earth & the actions people on earth take in worship of God:
14 The Lord shall increase you more and more, you and your children.
15 Ye are blessed of the Lord which made heaven and earth.
16 The heaven, even the heavens, are the Lord’s: but the earth hath he
given to the children of men.
17 The dead praise not the Lord, neither any that go down into
silence.
18 But we will bless the Lord from this time forth and for evermore.
Praise the Lord.
- If the last 2 verses speak of actions taken in the afterlife, they
contradict one another (i.e. after we're dead we won't praise ^ after
we're dead we'll still praise for evermore).
- If the last 2 verses speak of actions taken on earth they are quite
compatible: our people/children (see verse 14) will praise God on
this earth forever--but from our perspective (on earth) the dead go
silent--we hear nothing from them. The people who are praising God on
earth are the living people of Israel (notice there's no mention of
Sheol here).
Thus the Psalmist either believes:
- He will never die OR
- He will praise God for ever--including after he dies OR
- His people/descendants on earth will praise God forever
#1 conflicts with the message of the Psalm (to say nothing of many of the other Psalms), #2 would support the consciousness of the dead, and #3 (the most likely option) would mean he's just talking about actions on earth.
I suggest #3 fits best with the focus of the Psalm, contrasting the works of God in heaven with those of men on earth. In that case, the Psalmist is quite correct: dead corpses on earth are silent and do not praise God.
Psalm 6:4-5, Psalm 30:9, Psalm 88:10-12
These passages highlight the temporal nature of David's concerns. While his spirit is in Sheol, David will not be doing the things he's doing now (in life) to praise God and teach His message. He sees an end to his ability to do what God sent him (David) here to do.
In this Psalm, David is grateful to God for preserving his life on earth and wants to praise & serve God on earth in gratitude. I offered a more extensive discussion of two of these passages in this post on the Hermeneutics site.
Job
All 3 passages from Job raise questions that are later answered in chapter 19. Job is struggling with grief, but after much mental anxiety he ultimately shares his conviction that there is hope.
Isaiah 14:9-11
All individuals referenced in the passage appear to be wicked--they are not enjoying a blissful afterlife.
Isaiah 38:17-19
Death is contemplated with despair--that's a normal human attitude. Isaiah is grateful that his life has been preserved and that he can continue to teach God's message.
Ecclesiastes 9
I offered a more extensive discussion of these passages on the Hermeneutics site here. This chapter is focused on events “under the sun”, which refers to the things of this life. In the aforementioned post I also offer a reductio ad absurdum that if Ecclesiastes 9 is used to argue for post-mortal unconsciousness, it could be used to deny the resurrection just as well.
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Conclusion
There are innumerable reasons why a person may feel apprehension about death, or the process of dying. Even Paul, who spoke somewhat favorably about dying, defended himself vigorously when on trial for his life.
Despite their human nature, Job, David, and Isaiah all convey a message of hope through the resurrection, even if not every step of the path between here and there is blissful.