I propose two plausible answers to this question:
- We could infer that David & Isaiah held views similar to later Jewish writers...that might be true.
OR
- We could acknowledge uncertainty as to what they believed on topics on which they didn't speak.
Neither David nor Isaiah had read the New Testament [citation needed], so it is difficult to be certain how much of the New Testament's afterlife theology was known to them. (Note that it is possible for something to be both true and unknown)
It is possible to read Isaiah 14:9-10 as a description of consciousness by the wicked in Sheol; it is also possible to read it as a metaphor (or both!).
Commentary on the referenced 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
I find Gill's commentary on this passage well-stated:
For in death there is no remembrance of thee,.... Of the goodness,
truth, power, and faithfulness of God; no notice can be taken nor
mention, made either of the perfections or works of God, whether of
nature or of grace, by a dead man to others; he is wholly useless to
men on earth with respect to these things; in the grave who shall give
thee thanks? for mercies temporal or spiritual; the dead cannot praise
the Lord among men, only the living; see Psalm 30:9; wherefore the
psalmist desires that he might live and praise the Lord: this argument
is taken from the glory of God, which end cannot be answered among men
by death, as by life. It does not follow from hence that the soul
either dies or sleeps with the body, and is inactive until the
resurrection morn, neither of which are true; or that the souls of
departed saints are unemployed in heaven; they are always before the
throne, and serve the Lord day and night; they remember, with the
utmost gratitude and thankfulness, all the goodness and grace of God
unto them, and praise him for all his wondrous works: but the sense
is, that when a saint is dead, he can no more serve and glorify God on
earth among men.
While 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.
--
Psalm 88:10-12
This says nothing about consciousness or unconsciousness of the dead, but does perhaps suggest the futility of procrastinating turning to the Lord. It asks a question which is graciously answered elsewhere--yes indeed the dead will rise.
See a more extensive related discussion in my comments on Ecclesiastes 9 here.
--
Isaiah 38:18-19
Verse 18 describes despair, not consciousness.
See also the final paragraphs (above) in the discussions of Psalm 115 & Psalm 6.
Conclusion
David & Isaiah tell us relatively little of their afterlife theology, but they do speak of Sheol, which can be understood through the words of later Jewish & Christian writers.
JewishEncyclopedia provides the following helpful statements on Sheol:
- It connotes the place where those that had died were believed to be
congregated
- [Sheol] seems to have been viewed as divided into compartments...one
beneath the other
- Here the dead meet.
- In the compartments of Sheol, one is often presented as a place of
rest for the righteous, and one a place of anguish for the wicked.
This is quite compatible with the view of the afterlife described in the parable of the rich man & Lazarus in Luke 16. This may be what David & Isaiah believed, or it may be that in their time some of this had not yet been revealed.