The State of the Dead
When a person dies they are considered to be "asleep".
These things said he: and after that he saith unto them, Our friend Lazarus sleepeth; but I go, that I may awake him out of sleep. Then said his disciples, Lord, if he sleep, he shall do well. Howbeit Jesus spake of his death: but they thought that he had spoken of taking of rest in sleep.
- John 11:11-13 (KJV)
The dead "know nothing".
For the living know that they shall die: but the dead know not any thing, neither have they any more a reward; for the memory of them is forgotten.
- Ecclesiastes 9:5-6 (KJV)
The dead will be raised on the last day.
For the Lord himself shall descend from heaven with a shout, with the voice of the archangel, and with the trump of God: and the dead in Christ shall rise first: Then we which are alive and remain shall be caught up together with them in the clouds, to meet the Lord in the air: and so shall we ever be with the Lord.
- 1 Thessalonians 4:16-17 (KJV)
So it's all pretty clear. The dead are asleep until the last day.
That's why mediums are so dangerous, because they talk directly to demons.
Regard not them that have familiar spirits, neither seek after wizards, to be defiled by them: I am the Lord your God.
- Leviticus 19:31 (KJV)
The phrase "familiar spirits" refers to mediums who have spirit guides, etc.
This kind of deception will only get worse in the last days.
He [Satan] has power to bring before men the appearance of their departed friends. The counterfeit is perfect; the familiar look, the words, the tone, are reproduced with marvellous distinctness. ... Many will be confronted by the spirits of devils personating beloved relatives or friends and declaring the most dangerous heresies. These visitants will appeal to our tenderest sympathies and will work miracles to sustain their pretensions.
- Ellen White, The Great Controversy p. 552, 560
It's also interesting to see that Christians will never see death, e.g. they will "die", but they will then awake from their sleep.
Verily, verily, I say unto you, If a man keep my saying, he shall never see death.
- John 8:51 (KJV)
A clear distinction between sleep and (permanent) death.