How we can believe in something even when we can’t see it for ourselves

Another example is the work by the Chinese artist, Song Dong, who keeps a personal diary written in clear water on stone, leaving inscriptions that rapidly evaporate. He began this bizarre practice in 1995, when he realised that conventional diary-writing posed the risk of exposing his most intimate thoughts and feeling to others.

It was Freud's fervent hope that civilisation would evolve beyond this infantile stage and science would replace all religion

Seventeen years on, after pouring out his most intimate thinking in this way, he believes that the stone - which to the observer betrays nothing of his activity - has actually become thicker every day with the accumulation of his thoughts. In the light of this information, it is impossible for the viewer to see the stone as simply a stone. One invariably projects this awareness on to the blank surface of the stone, seeing in it something else entirely; the reflection of a man's intimate thoughts and feelings.

Much of how we perceive the world depends on what we expect to see. The psychiatrist and neuroscientist Ian McGilchrist makes this argument in his book, The Master and his Emissary, in which he unpacks the workings of the two hemispheres of the brain. Each new experience, as it is first presented to the mind, he writes, engages the right hemisphere. Once the experience becomes familiar, it gets re-presented by the left hemisphere and that re-presentation is very much guided by what we expect to see.

This is why, to give an everyday example, we can drive along the same road on the way to work for years and yet still not "see" unexpected changes along that road such as new street furniture. The first time we drove that route we saw things as they really were. On subsequent journeys we see what we have come to expect.

Ultimately, much of what we see can only be described as a grand illusion, since we tend to experience more and more only what we already know. Ultimately, McGilchrist concludes, we become so entrenched in this cycle that it is hard to know how we can ever come to experience anything truly new.

Wandering around the gallery and engaging with the artwork it suddenly occurred to me that the influential 19th-century atheist philosopher Ludwig Feuerbach would have felt right at home wandering through the Hayward Gallery.

Feuerbach famously asserted that there is no independent reality called God. Instead, he said that what we describe as God is nothing more than humanity projecting its deepest values and loftiest ideals onto an imagined deity.

God, for Feuerbach, is no more real than the fantastic images we choose to project on to Jakob's blank canvass. It is all a figment of our own overactive imagination and we would all be better off focusing less on contrived heavenly attributes and more on human justice and compassion. Years later, Sigmund Freud similarly argued, in The Future of an Illusion, that the illusion of God is one of the most important items in the psychical inventory of a civilisation.

It was Freud's fervent hope, however, that civilisation would evolve beyond this infantile stage and that science would replace all religion and reason would replace faith in God.

While Feuerbach and Freud argued that man should evolve beyond the primitive stage of religious belief, pragmatists like William James maintained that belief in God can be justified regardless of whether or not He actually exists. This is because such belief - which can neither be proven or disproven - can be immensely useful in the way it brings about extraordinary positive changes in people's lives. A contemporary of William James, the American psychologist James Leuba, likewise held that proof of God's actual existence is irrelevant to a person of faith.

He wrote: "God is not known, he is not understood, he is used sometimes as meat-purveyor, sometimes as moral support, sometimes as friend, sometimes as an object of love. If he proves himself useful, the religious consciousness asks for no more than that. Does God really exist? How does he exist? What is he? are so many irrelevant questions. Not God, but life, more life, a larger, richer, more satisfying life, is, in the last analysis, the end of religion."

True beliefs, then according to James and Leuba, are those that are useful to the believer. In fact, in response to a survey about religious belief, James indicated that he believed in God, not because he had any evidence of God's existence but because he needed such a belief.

And yet, just because the canvas is blank it does not mean that it is intrinsically empty. Simply because a reality is incommunicable does not necessarily undermine its veracity. The mystics also thought of God as a blank canvas. They believed that any attribute or quality that one ascribes to God is nothing more than our description of how God is manifest to us.

As for God Himself, nothing can be said, since He transcends all description and characterisation. In fact "He" is neither he nor she, simply the En Sof; the infinite, the indescribable, the inscrutable. Any attempt at definition is detraction. Any characterisation is to limit the limitless. The mere use of language is severely inadequate, which is why the mystics struggle with language when talking about the divine.

Gershom Scholem pointed out how the seminal text of Jewish mysticism, the Zohar, is filled with oxymorons and paradoxes for this very reason. According to Scholem, when the Zohar employs a term such as: "it is and is not," it is not struggling to describe something that exists only partially but rather that its existence is of such a lofty spiritual nature that it cannot be properly described.

Even though nothing rational can be said about God, it does not mean that belief in God is inherently irrational. The American psychologist and philosopher Ken Wilber makes an important distinction between what he calls pre-rational and trans-rational states of mind. In the pre-rational state, one has not thought the matter through and is simply "irrational". Yet in the trans-rational state, one recognises that rationality has its own severe limitations and therefore seeks to go beyond these limitations.

This awareness of something divine beyond language and description is, according to the Lutheran theologian Rudolph Otto, at the heart of all religious faith and the feeling it engenders is what he calls "the Numinous", which he describes as "a non-rational, non-sensory experience, or feeling whose primary and immediate object is outside the self".

He also refers to it as the mysterium tremendum et fascinans, a great mystery that invokes both fear and fascination, the operative word here being mysterium. Rabbi A J Heschel, who was influenced by Otto's thinking, argued that the rational mind cannot be the sole arbiter of reality.

In his study, Man is not Alone, Heschel writes that: "The most basic objection to the belief in the existence of God is the argument that such a belief passes from the mind's data to something that surpasses the scope of the mind. What gives us more assurance than an idea which we may find ourselves obliged to think may hold true of a reality that lies beyond the reach of the mind? Such an objection is valid when applied to the speculative approach.

"Yet, as we have seen, the certainty of the existence of God does not come about as a corollary of logical premises, as a leap from the realm of logic to the realm of ontology, from an assumption to a fact. It is, on the contrary, a transition from an immediate apprehension to a thought, from being overwhelmed by the presence of God to an awareness of His essence."

The mystics and Feuerbach agree up to a point, though. They both see the characterisation of God as a human endeavour, not dissimilar to projecting images on to a blank canvas. Where, however, they part company is in their conception of the blank canvas itself. While Feuerbach contends that it is nothing, the mystics maintain that it is everything. For Feuerbach, there is no independent divine reality beyond our own imagination. For the mystics, God is absolutely real even if at the same time He is inscrutable.

The practical upshot of all this is awareness that despite our best efforts we cannot possibly know God. While this seems obvious to some, it is sadly not the case for others. A decade into the 21st century, and we are witnessing a resurgence of faith.

While on the surface this can be seen as a good thing, it is the kind of faith professed by many that is disturbing. It can only be described as a "faith of arrogance", a faith that purports to know exactly what God is and what He does, and does not want, leading its proponents to act as God's enforcers.

Such confidence and certainty can be toxic when aligned to man's baser instincts, including the craving for power, greed and self importance. Sadly, no faith has a monopoly on this dangerous distortion. We see it and read about it all the time. What frequently passes for religious zeal is barely masked opportunism, in which individuals project their own selfish needs, desires and fears on to God.

We need to rediscover the wisdom of the mystics and start putting forward a faith of humility that is underpinned by awe and wonder at the infinitude of the divine - the mysterium tremendum - while at the same time acknowledging our own severe limitations and inadequacies. The result is to proceed humbly and cautiously on one's religious journey as the prophet Micah (6:8) said - "Walk humbly with your God".

A visit to the Hayward Gallery is a rather useful first step in this direction.

Naftali Brawer is chief executive of Spiritual Capital Foundation. 'Invisible: Art about the Unseen,1957-2012' is at the Hayward Gallery until August 5