It is very strange that there are so many humans who do not know, do not believe or question the existence of the soul ; who doubt that there is within us an immortal soul, or at least, some principle which transcends our short-lived physical existence.
We can assume that there are several reasons behind unbelief or a form of doubt in the existence of the soul. One of these reasons is non-adherence to any system of religious beliefs. Now, complete or partial adherence to any system of religious beliefs is not at all related to knowledge or ignorance of the fact of the soul. The soul is not something religious nor (perhaps) something mythological, it is a natural, fundamental entity. It is intrinsically part of the nature of things. Whether religions exist or not, the (spiritual) fact of the soul is, has always been and will always remain. Even if the religions of the world deal (more or less correctly) with the subject of the soul, there is no need to know or to adhere to any form of religious belief to become fully aware of the fact of the soul.
Another reason behind unbelief or a form of doubt in the existence of the soul comes from the fact that, precisely, one is not conscious of the fact of the soul. But behind this unconsciousness lies a reason which should be obvious: one does not seek to become conscious of the fact of the soul. Maybe because we don't believe it exists, but most likely because we don't know how to go about it. Now, becoming conscious, beyond all doubt, of the fact of the soul is entirely possible, if we take the time and make the necessary efforts in order to realize what the nature of consciousness is.
The fundamental reason why we do not know the soul is therefore essentially a question of consciousness, a question of what we are conscious of. It is not a question of what we believe. In reality the terms "consciousness" and "soul" are essentially synonyms. Terms like "attention", "concentration", "alertness" refer to the periphery of what the soul is.
Perhaps unbelief stems from the (apparently) problematic notion that the soul is immortal? But if one strives to discover and realize what the nature of consciousness is, this problem disappears by itself. This apparent problem comes from the fact that the consciousness which has not realized what its own nature is, is constantly occupied and encumbered by the impressions coming from the body, desires and emotions in addition to the thoughts which are constantly raised in it, before disappearing, replaced by others. All of these things are entirely fleeting, temporal, whereas consciousness itself is not.
Thus, the practice of attention or constant vigilance, the refusal to be distracted or dispersed, the practice of meditation (which is essentially the practice of attention), allow in the long run a clarification of consciousness, a gradual withdrawal of these temporary and temporal things which encumbered it so that the nature of consciousness becomes clear and obvious. The term "immortal soul" sometimes used really means "timeless" - that is to say "that exists beyond time". The impression that there is such a thing as time passing by comes from the constant change of these things of which we are conscious of (impressions, sensations, desires, emotions thought) things that are necessarily impermanent. If consciousness is largely rid of these things which only exist in time, its nature appears as something which is beyond time and does not change. Apart from change, there is no time, in the ordinary sense given to this term. There is only eternal or timeless present. Being fully attentive is, after all, only possible in the present; it is nonsense to be attentive outside the present moment.
To conclude, non-belief in the existence of the soul is not so much more valid than irrational or blind religious belief. The soul has nothing to do with believing but has everything to do with the will to be conscious, to practice conscious attention (or meditation) at all times. The consciousness that can distinguish between what is its own nature and what is not part of it, will eventually come to know itself as it is in its reality and its entirety: as an entity who experiences this world, but who is not of this world; who experiences time, but who is beyond time; who observes many passing phenomena, but who is not passing and invariably remains what it is.