Who is Satan?

There is a very real difference between 'diabolos' and 'satanas'. And 'the Beast'.
For Steiner, they are respectively: Lucifer, Ahriman, Sorat.
they are very high spiritual beings, and they govern fallen hierarchies and elementals, which do not have an ego of their own and therefore cannot make moral choices between good and bad ; they merely oscillante between sympathy and antipathy and acat according to these basic astral egoless drives.
The inner earth is filled with spiritual, as everything else. We find there an inverted natural world, and the more we go toward the center, the more evil we find. there are cosmic hierarchies and they all have a fallen counterpart inside earth, each stratum corresponding to a particular class of fallen cosmic hierarchies. The subterrestrial world, as with elementals and fallen elementals are not evil, as I said they act upon sympathy and antipathy without the freedom we do have. They can steal parts of our ego thru taking out our fluids, blood and hormonal (eheric) fluids for instance and can act upon our nervous system (astal). As the gods of the underworld are not evil, Osirs god of the underworld is not evil, he is a face of Christ as an interior god, god of the interior of the soul. Gods that are outside in the natural world and cosmos are called higher gods. Egypt stuck with inferior and gods of the underworld, of the inner soul of man and afterdeath world, but Chaldea-Babylonia stuck more with gods of the outside world, in astro-theology, mapping out the cosmic spheres and their corresponding higher gods.  In general, gods of the interior of the soul, inferior gods, gods of the underworld, are called luciferic. But also Christ and Yavhe are gods of the interior of the soul, lower gods. Solar gods are the higher ones, and the lunar ones are the lower ones. Beings who tempted us in the Garden where once higher solar gods, but they couldn't bear living on the sun because they retardated their development since ancient moon. Thus they had to extract Venus (as with mercury) from the sun. They also fell on Earth later, living alongside kindergarten man but as higher superhumans semi-angelic incarnate beings (later in Atlantis they weren't incarnate, they just manifested in fiery etheric body at the culmination of initiation in hidden mystery centers in Atlantis). These fallen luciferic beings who were once solar beings, became moon gods, linked with our soul. Yahve is a different kind of moon god: he sacrificed himself in order to mineralise the moon in order to keep atavistic old moon influences (old moon influences and substance were and still are co-opted by Lucifer and Ahriman) away from earth, to make a counterpart to the eighth sphere in order to neutralise it.  Yahve was also an interior god, a true progressive lunar god, creator of our body image and giver of kama-manas, prime sense of egohood, but opposite to the influence of retardated luciferic beings. Christ was from the beginning a god of the soul of Man. But parallel to the fall of Lucifer on ancient Sun, Christ took upon himself the karma of the World and united himself in Love with Earth development and Cosmos.  He thus became the Logos, the Prime Model of the Universe, the Rule, the Principle, which shapes the Cosmos (which means order as opposed to chaos). This made him a higher god, a god of the outside world, no more of the inner soul of man, for a while. He was part of these solar cosmic gods described by Zarathoustra, he was Ahura-Mazdao and his spirits of Light, as opposed to Ahriman and his spirits of Darkness. Thru his Incarnation and thur the mystery of Golgotha, Christ became a human god, a good linked to our inner sul. Thus the mystics looked for Christ inside us. The thunder was no mokre outside man in a fiery bush Yahvw, he waas to be found inside, deep inside, as a I-God, god of higher self, but lower god as it was a god found after death and in the inner world of the soul. It is also the Mystery of Osiris-Christ who undertook the trip inside the underworld. The fragmentation of Osiris means the splitting up of global Man in fragmented individualities and sense-perception. We dont have to search for Osiris anymore because the mystery of Golgotha resolved this quest: Christ united himself with Earth thru His blood falling on the soil of Mount Golgotha, as inverted chalice. It is the Wisdom of Sophia that is lost, the knowledge, the sophia of Christ that is lost to man, not Christ himself. Christ is already back in the astral realm and his infinitely multiplying image is to be found at will in the etheric realm. What we miss is the knowledge of Christ, this is the mystery of the New Sophia. The New Isis Mystery. This is miles away from any 'anti-monotheist pagan witchcraft Goddess religions' come back! Because the connection with nature was once based on atavistic clairvoyance, underdevelpped individuality, unfree human beings. The new clairvoyance comes with a heightened sense of self, not thru blissful annihilation of ego-conscisouness in saturnalia bacchic exctatic (even druidic) orgies and chamanic trance.   

There are many references on these spiritual beings Lucifer-Diabolos, Ahriman-Satan, SVRT-Sorat (the Beast 666), works by Steiner and other more recent anthroposophists. I quoted many in the beginning of the 'food for the eight sphere, not the moon' thread.