The doctrine of the immortality of the soul is one of Satan's first lies to the human race.
"And the woman said to the serpent, 'We may eat the fruit of the trees of the garden; but of the fruit of the tree which is in the midst of the garden, God has said, "You shall not eat it, nor shall you touch it, lest you die." ' Then the serpent said to the woman, 'You will not surely die. For God knows that in the day you eat of it your eyes will be opened, and you will be like God, knowing good and evil' " (Genesis 3:2-5).
Satan and his demons continue this lie among religious people, though they also teach the lie of materialism to atheists and those inclined to believe there is no God at all. Satan and his demons continued this lie in the time of King Saul, deceiving him into thinking that he was seeing the ghost of Samuel - see my post titled, "Why Did God Kill King Saul and Turn the Kingdom over to David?", dated April 20, 2013, link:
There is an obvious reason why Satan promotes this lie. This idea, that each of us will exist forever, that we cannot really die and cease to exist, combined with the teaching that no one except those who hear the true gospel and are saved in this life by accepting Jesus Christ can escape condemnation to hell fire, teaches that God is a cruel monster that tortures immortal souls in hell fire for eternity, and that most of humanity falls into this category. It is a slander against God. It makes God appear weak or unjust or unloving. It is one reason why many materialists reject all religion and belief in God. See my post titled, "The World's Loved Ones Are Not Lost", dated September 29, 2010, link:
But this lie does additional damage to religious believers and to understanding God's truth. Because many believers in God's existence think that the alternative to being in God's kingdom is suffering forever in hell, and because they may think that God is still compassionate and loving, many may tend to think that God will find a way to accept everyone or nearly everyone into His kingdom, even without real repentance and obedience to His word and commandments.
Thus, the lie of the immortal soul helps members of traditional churches believe that God is lax when it comes to obedience.
In effect, while they do not paint God as cruel, they paint him as a liar, as one who does not mean what He says, like a parent who warns his or her child of punishment, "If you do that again, I will spank you", but does not follow through (and the child knows the parent will not follow through, so he continues to disobey). So they think God doesn't mean it when He teaches throughout the Bible that only those who believe and obey Him will be accepted into His kingdom (which traditional churches think is heaven). They do not think God will cast very many into eternal torment, immortal souls being tortured for all eternity with no hope of release, for something as "innocent" as keeping Christmas to honor Christ, even though God teaches us that we should live by every word of God and the Bible teaches against Christmas.
In effect, they think that God is "bluffing" in the Bible, that he does not really exclude people from his kingdom and cast them into hell for disobedience as long as they are mostly good and follow their conscience, and this teaches them, by example, that it is ok to tell "white lies" to their children as they think God tells them lies in the Bible.
Thus, parents think it is ok to tell the lie of Santa Claus to their children, because they think it is good for their children because it is fun. In this, they follow the example of the way they think God is, the way Satan paints God, as not meaning what He says.
Thus, Satan's lie about the soul being immortal is a double-trap. People who swallow this lie can fall into one of two ditches. If a person is inclined to disbelieve the existence of the God of the Bible (atheists do not believe in God because they do not want to believe in a God who has authority over their lifestyles, who tells them what to do and how to lives their lives), they will use this doctrine to conclude that the idea of the God of the Bible is ridiculous, because only a cruel God would torture the majority of mankind for eternity. But those who want to believe in God (because they enjoy thinking of themselves as righteous, and belief in God is part of their self-image they want to protect), can simply reconcile their idea of God being a God of love with their belief in the immortal soul by concluding that God will not really cast the disobedient into hell but will accept them into His kingdom anyway, even though they do not really believe and obey him, as long as they have good intentions.
Thus, religious people will think God will accept their good intentions about "honoring Christ" with Christmas even though they may acknowledge in their minds that God says not to do it. They tell themselves God doesn't mean what he says, that obedience to God's word is really not that important to God as long as you have good intentions and try to be a "good person".
But God will not accept them into His kingdom as long as they have not repented of their disbelief and disobedience in His word. God will not have another Lucifer in His kingdom and family. He will not entrust eternal life to someone who thinks that He is bluffing in His word, that He doesn't really mean what He says, that He gives warnings but doesn't back them up. Because if He gives eternal life to someone like that, then sooner or later that person, in the eternity to come, will doubt and disobey God's word, rebelling against God and ruining the happiness that could exist in God's family for eternity. God will not allow that.
One of the reasons we can look forward to God's kingdom is that we know that everyone in it will be totally submitted to God in their character because they will have proved in their human lives, in this life now if they are called to be part of the firstfruits, or in the millennium, or in the white throne judgment period, that they will totally trust, believe, and obey God forever. That is why the Kingdom of God will be happy, and that is why we can look forward to an eternity in that kingdom without any fear of another Lucifer rising up and causing rebellion and war.
Satan's lie, that man has an immortal soul, destroys any foundation of right character building that is necessary to prepare someone for God's kingdom. In a multitude of ways, it does damage to a right understanding of God and His way of life and the purpose of human life. God allows Satan to teach this now, to deceive mankind so mankind writes the lesson in human experience that Satan's way brings suffering, but when Satan is put away, the deception will end, and God will begin to teach mankind the truth. Then every person can make his decision to believe and obey God or not, free from the enslavement of Satan's deceptions.
Here are links to related chapters or sections in Preaching the Gospel:
The Day of Trumpets - the Second Coming of Christ, Chapter 2
Traditional Christian Holidays, Chapter 2
Faith, Chapter 6