And they will make a new world, freed from old age and death, from decomposition and corruption, eternally living, eternally growing, possessing power at will, when the dead will rise again, when immortality will come to the living, and when the world will renew itself as desired. Yt19.11 
This passage, one of the most beautiful to be found in the younger Avesta, proclaims at once with missionary zeal the goal towards which all Zarathushtrian efforts were directed: nothing less than the total transformation and perfection of existence (Frashkart).
Zarathushtra s vision of the supreme deity, Ahura Mazda, was that of a good God: wholly benevolent, totally loving, the author of all quality, beauty, and of everything life-enhancing and positive. But precisely because he was entirely good, he was not all-powerful. He possessed a vulnerability that was an attribute of his goodness, a vulnerability present in all those who are sensitive and benevolent. Ahura Mazda desired Man to participate with him in bringing the creation to perfection. (Y31.21). Man was free to accept the call or to refuse it. The invitation was given freely, without any threats of punishment or promises of reward: the end result would be reward enough. The process towards its completion: a journey of creative self-discovery in which the individual would both finds and fulfills his intrinsic humanity.
Answering this divine call, the followers of Zarathushtra saw themselves as a loose brotherhood of individuals working (each on his own initiative) in a common cause alongside their God to transform the world -- their thoughts, words and actions reflecting the longings of their prophet:
May we be among those who bring about the transfiguration of the earth. Y30.9
The means by which this desired end was to be accomplished was by an ever-greater growth and evolution of the light of glory (xvarnah). This primordial light, uncreated because it was a natural property of the deity (Ys 12.1, 31.7, 35.10), was the energy out of which Ahura Mazda had created everything in existence including the divine beings of the pleroma. It was a light that filled the heavens, the abode of light (Y31.20) but was undeveloped and latent in matter. The basic duality in the philosophy of Zarathushtra was not that of light against darkness, but of manifestation and latency of the light (the menok and getig states of existence). In addition, the word xvarnah carried with it the implication of destiny, suggesting a positive bias in the universe towards the emergence and evolution of the light - a kind of anticipation of the Frashkart at the heart of creation: an assurance, similar to that of Julian of Norwich that, all shall be well..... and all manner of things shall be well.
But this was not the purely abstract light of the Gnostics and Manicheans. It was not an alien presence imprisoned in the grossness of matter, calling out to the individual to free him out of the stinking body out of this desolate place. 
The light did not require the individual to reject matter or retreat into the rarefied world of the intellect. This radiance was an intrinsic property of matter. Man belonged to the earth and the earth belonged to Man. He would never be able to feel himself at home anywhere else but in the material world. The Zarathushtrian conception of this interrelationship of man with nature was very strong. Man was not placed into the universe like an object among other objects in the way that the God of the Old Testament placed Adam into an already-completed garden. Rather he was born out of his environment like an apple from a tree, or ripples from a pond.
Hence arose the Zarathushtrian respect for all life and nature, a reverence which the prophet himself voiced in his songs:
The radiance of the sun and the shimmering of the dawn at the break of day are reflections of your glory Y50.10 and which his followers echoed:
We revere all the holy creatures that Mazda has created, which were established holy in their nature and we revere all the springs of water and the growing plants and the entire earth and heavens even towards the lights without number. Y71.6
When he looked about him at the physical world, the Zarathushtrian was confronted by the goodness of Ahura Mazda reflected, in some form or another, in every object and being which he saw; and (in the later literature) each element of the physical world was imagined as under the protection of one or other of the Amesha Spentas (Holy Immortals), the hypostases of Ahura Mazda.
The final transfiguration of the world, its final ideal state (Frashkart), is an image of the universe ablaze with the auroral light of the xvarnah, a light illuminating all things animate and inanimate, bestowing meaning and value upon them, and opening up their dimension of transcendence.
Seeing Persons, Lighting Haloes
It is all too easy to imagine the xvarnah, so seemingly abstract and distant from everyday life, as merely some fanciful metaphor with relevance only for poets and philosophers. But this light of glory, about which so many books have been written, is exactly what makes each of us uniquely human. Its influence can be discerned in all the minutiae of human life. In order to gain a real feel for the benefits of the light, it is necessary only to consider the concept of the person.
Most of us are able to experience a human being in one of two ways: as an object (a collection of tissues, chemical processes and electrical impulses) or as a person (an indivisible whole with a face and a name). Once an object is perceived as a person, a mysterious new dimension opens up: we recognize something that exists on a higher level than mere sensory perception. We respond to the infinite within the finite. To recognize a person when all we have before us is a mass of physical characteristics - hair, teeth, tissues - is to perceive that object qualitatively, i.e., to see it in a new light: in the light of the xvarnah.
We take for granted our remarkable ability to perceive persons: we hardly give it a second thought. Its sheer wonder becomes clear only once we experience someone who possesses no such intuition: the classic autistic person. Broadly speaking, the severely autistic individual can be described as being trapped in a world of physical matter and strict reasoning. He finds it difficult to communicate, to imagine or to deal with other people socially. Although good at learning complex rules, he is nevertheless incapable of reacting sympathetically to others because he can never imagine what anyone else is thinking: he has no concept of mind. The whole interior (infinite) world of the person as person is unknown to him. His relationships are directed chiefly towards objects: which is how he perceives other people -- as objects.
Science knows nothing of the person. The person (the uniqueness of the person) cannot be expressed in concepts at all. It evades all rational definitions because all the properties by which it could be characterized can be met with in other individuals. Personality can be grasped only by direct intuition. Similarly, a face - the symbol of the person - differs from all other faces in very minute details, barely describable in words. Yet to other human beings, the recognition of these features as a unique person goes far beyond what science can explain.
Once the internal world of another individual is revealed, by virtue of our recognition of him as a person (an object open to infinity), the whole world of human relations suddenly becomes possible: co-operation, intimacy, compassion, understanding, love..... civilization.
When we fall in love with another human being, we are seeing that individual as more than just a person. For a time, the image we have of him or her is complete (because illuminated strongly by the light), whole, and hence (whole-ly) holy. That atmosphere of wonder and colour that suddenly surrounds the object of our attentions (when coincidences abound, when the world suddenly becomes saturated with meaning and everything in creation revolves around this single human being), is a quality of the xvarnah. We are loving someone who does not (yet) exist. We are seeing them as they will appear (one day) in the full light of the Frashkart.
The halo (the aureole, the nimbus) is one of the great abiding icons of Zarathushtrianism. This is the light which in Zarathushtrian as well as in Christian and Buddhist iconography, is to be found glowing about the heads of great kings, priests or holy men. Each of us has set at least one halo ablaze in the course of our lives. When we fall in love, it is as if we have lit up the beloved s halo. Perceiving their dimension of transcendence, we recognize the divine in them. For what is a halo but a human being lit up with the light of great love, value, or destiny? A lover does not love the physical body of his beloved at all, but the ideal image of her, the angel to whom she corresponds. Of course he does love her body also, but for the sake of her person: because it belongs to her and manifests her reality. That physical body can be old as a grandmother, sick, diseased, (barely recognizable as a human being), punctured by tubes and plugged into monitors, but still loved and adored for the person within it.
The divine light, the sacred fire, the so-called fire-temples, the aureoles, the Mountain of the Dawns, the Peak of Judgment, the Auroral fires, the Chinvat Bridge - the symbolism can easily become heady, the imagery intoxicating. Reason begins to lose its foothold here. But to remove such poetic elements at the heart of any philosophy or religion is to rip out its heart and corrupt its truth. For religions, as well as philosophies, live and breathe by the quality of their poetry: by their ability to set hearts alight and not just heads. Sometimes it can be more instructive to follow the images of thought to where they lead us, rather than rush immediately to dissect with the intellect. (We are reminded that Zarathushtra was first and foremost a poet, and proud of it). One of the utterances of the Delphic Oracle was that only poetry could be accepted as truth in every age.
So many traditions bear witness to the experience of the uncreated light that it is impossible to indicate even a tiny representative sample here. It is the fire of the Burning Bush seen by Moses; the pillar of fire before the Israelites in the desert. It is the Kibriya. It is the tongues of fire revealed to the apostles of Christ at Pentecost; and the light of the Transfiguration glimpsed by them on Mount Tabor (Lord, Lord, this is a good place to be). The Manichaeans, blinded by its beauty, looked in disgust at the material world that had become dark and dead for them in contrast. Rumi wrote eloquently in praise of it, but at first he was terrified of its illumination:
I lost my world, my fame, my mind
The sun appeared and all the shadows ran
I ran after them but vanished as I ran
Light ran after me and hunted me down 
The Islamic philosopher who borrowed more elements from Zarathushtrianism than any other was probably Suhrawardi. For him the universe was an infinite sea of lights: nothing existed that was not light. It is interesting to note that Suhrawardi reserved a special place for Vohuman in his Philosophy of Lights. Whereas he equated the other Amesha Spentas roughly with Plato s Archetypes (his latitudinal order of lights), Vohuman (Bahman) he considered the primary archangel of the longitudinal order: the first light emanating from the Godhead, the nearest to the supreme Godhead himself (Hormuzd).
All truly great symbols overflow the boundaries of meaning and invade the world of the senses. Light (perhaps the greatest symbol of them all) is no exception. Many of the early Christian saints such as Gregory Nazianzen, Cyril of Alexandria, Maximus, Macarius of Egypt, Andrew of Crete, John Damascene, Symeon the New Theologian, Euthymius Zigabenus, etc., all spoke of the Divine Light as if they had seen it with their bodily eyes. For if the intensity of the light is in some way a measure of wholeness, some argued, then surely it should be experienced by the whole man and be perceptible to the physical senses as well as to the intelligence. I had often [bodily] seen the light , wrote Symeon the New Theologian in the eleventh century in defence of this position - and we have to believe him. But the dispute as to whether the light could in fact be seen with the bodily eyes split the Christian Orthodox Church. Gregory Palamas (the Byzantine apostle of light) healed the rift in the fourteenth century with a series of compromises, but he still remained tantalizingly ambiguous on the subject:
The light has sometimes also been seen by the eyes of the body, but not with their created and sensory power; for they see it after having been transformed by the spirit. 
Yet the basic intuition of a synchronism between the spiritual and the sensual continued to be felt and expressed. Writers like Rumi and Ibn Arabi conceived of the spiritual and the sensual as conspiring together in some mysterious and irrational fashion. And Suhrawardi, when defining his fifteen varieties of spiritual light, seemed often to be describing what are known today as photisms: intense flashes before the eyes sometimes experienced by people who practice meditation. Varying in intensity from pinpoints to large areas of bright and coloured lights, these photisms have been experienced by far too many people for them to be easily dismissed. Individuals as varied as Ibn al-Arabi and Emanuel Swedenborg have investigated them, (the latter thinker believing them to be internal signs of approval).
Is the divine light then purely intellectual; is it spiritual, physical; or perhaps all three? Is it a property of the very nature of God, or merely his energy? In the end, the real nature of this light defies all attempts to grasp its full significance, because you cannot demonstrate that which is itself the cause of all demonstration.
Yt19.11 in Corbin, H. Spiritual Body and Celestial Earth. Taurus 1976 (1990) pp. 13-14
Mandaean text in Jonas, Hans. The Gnostic Religion. Beacon Press p.88
Rumi in Harvey, Andrew. The Way of Passion. A Celebration of Rumi. Souvenir Press 1994 p. 59
Lossky, Vladimir. The Vision of God. (Trans. Ashleigh Moorhouse) The Faith Press. American Orthodox Book Service 1963 p.118.
Palamas, in Mantzaridis, Georgos. The Deification of Man. St. Vladimir s Seminary Press (New York) 1984 p.100