Over recent centuries in the West, there has probably been no change more profound than the loss of belief in souls, including the loss of the assumption that souls were eternal.
In the past this was taken for granted.
Indeed, it seems that a belief in the soul is universal, spontaneous, and rational.
There was disagreement about what became of souls (whether they were recycled, became ghosts, went to another place, became godlike...); but there was no doubt to our ancestors (and indeed probably most people in the world today) that souls were real and survived beyond death.
It is remarkable that this universal assumption has - somehow - become so ridiculous as now to be literally incredible among mainstream intellectuals.
(That was certainly how I saw things for most of my adult life.)
Why such a seismic change? After all, there was no discovery of the non-existence of the soul, no discovery that soul was an incoherent concept - there was merely a reversal of assumption.
And since the soul is disbelieved, modern secular people do not not worry about what happens after their death - they see death as merely going to sleep then not waking-up.
In the past, people would worry about what would happen after they died - because it seemed to them that their souls would be around for a lot longer than their bodies. Indeed, it was usual to be terrified about what might happen to the soul after death - that its fate might be horrible.
Belief in the soul was certainly not a consoling one; or not usually.
But secular moderns, disbelieving in the soul, assume that a belief in the soul is a species of wishful thinking - that people believe in the soul merely because they cannot face the reality of extinction after death.
Moderns regard those who believe in the soul as weak, or feeble minded, or deviously manipulative; to be pitied or despised but not to be admired or emulated.
But someone could only imagine that a belief in the soul - in 'life after death' was intrinsically a consoling fiction if one didn't really believe it.
The soul can only be a consoling concept if it is fictional and evanescent - to regard souls as factual and permanent is to be faced with a major consideration which would tend to dominate life.
So, on the one hand, when someone really believes in the survival of the soul and is capable of reasoning and imagination, then this is worrying; and to believe in the immortality of the soul is nothing less than a terrifying prospect.
Perhaps it is, therefore, more rational to reverse the modern secular assumption and to regard disbelief in the soul as the soothing, consoling fiction, the belief of those who cannot face the terrifying prospect of an uncertain state of immortality.
Also, modern secular intellectuals may need to acknowledge that they are deluded in their subjective disbelief in soul.
They (and I was one of them until recently) really sincerely disbelieve, and this disbelief seems intractable - yet rationally they must recognize that their subjective conviction is (at least highly probably) wrong, and souls are real.
Secular intellectuals therefore need, at least, a course in self-cognitive therapy. Sometimes this can shake a delusion. At root, this is a training in habit.
But even if by their best efforts they cannot shake the delusional disbelief, they still need rationally to acknowledge its delusional status.
Maybe it will help if they recognize that belief in soul in not consoling, but profoundly worrying - maybe belief in soul will then seem less pitiful and more heroic?
However they cross the delusional divide - we need to see that the collective spontaneous wisdom of mankind has been that souls are real and survive death, and are probably immortal - and this is a starting point - a given - for understanding the world and our place in it.
The reason why acknowledging the reality and probable immortality of the soul is a necessary first step for understanding human existence is that otherwise human society, its institutions and civilization (art, science, philosophy etc.) will be valued more highly than the individual - because they outlast the individual (as well as being stronger than the individual).
Indeed, we came come to value the environment or the planet more than humans or human society, since the earth will outlast humanity.
Whereas, in reality individual souls will outlast all forms of human society; will outlast the planet, will survive even the death of the universe.
This dizzying recognition was normal for thoughtful humans in the past, but has now been lost from mainstream discourse.
Indeed it is likely that a typical modern intellectual (to whom the soul is a childish superstition) would go through life without ever encountering this basic underlying perspective; a perspective once all-but universal, and a reality that ought to underpin our understanding of the human condition.