Who is the Devil?
1): There is a hidden teaching which says that when God created the universe it was perfect, but because of this perfection it had no reason to change. It was perfect in all directions. It therefore threatened to collapse in on itself and disappear. This would have undone the universe which would have made God sad and bored. He therefore brought his firstborn principles before him, the Archangels; and asked which of them would be willing to descend into the material creation and to act as a "stop gap"; introducing asymmetry and preventing perfection from threatening creation. Lucifer was the most beloved of God and stepped forward. He agreed to this task even though it would make him a scapegoat, because he would prevent the perfection of any created thing. Since all created things have an urge towards perfection in the hope of returning to the bosom of the primordial unity and bliss, they would necessarily resent this action of Lucifer's.
In addition, because Lucifer loved God more than any other and because he was going to be sent to the position farthest away from God in his task, he decided he would perform another service for God that could only be performed from the position of the antipode. He decided that he would invent allure, and seduce all those who sought to move towards God away from him. This would have a double action: it would keep God's love for himself, as the first action of jealousy, and second; it would keep all the unworthy out of God's "hair." In this function, the devil would act as "trier" of purity and intention, with the ideal that none should pass into God's bosom.
Because Satan would be at the antipode of creation but standing between the fulfillment of perfection, and since God in his role of creator stands at the center of the creation, from the position of those things which are created, Satan would seem to have usurped the divine position of the creator. This has the result that the universe resembles a house of mirrors, and thwarts any efforts towards finding God. With the introduction of the devil at the antipode of creation, the universe becomes an exitless magic circle.
The foregoing is from an esoteric musical teaching.
2): How are we to relate to the Devil?
In the late 1800s there was a group of French poets who reasoned thus:
Since God and the Devil were engaged in a kind of military warfare, one should use military allegory to understand spiritual strategy. They reasoned that most persons are of very little interest to the Devil because while thinking to themselves that they are headed for heaven, by their own actions they are in fact headed straight for Hell. It is therefore unnecessary for the Devil to waste any of the energies of his demonic legion on them. They could be assigned very minor imps and demons who would guide them towards the right berth in Hell. These souls were also quite skinny -- providing very little fuel for the fires of Hell. To the demonic reasoning most of the world provided much quantity but very little quality. This was offset however by certain individuals who led exemplary lives or who performed holy actions known or unknown. These souls were headed directly towards the gates of heaven, even sometimes without knowing it, and especially without intention (which itself would be a disqualifying "sin"). In order to capture these types of souls, a kind of ambush needed to be set up. This required rather high-ranking devils to hide in ambush near the gates of heaven. Then, after the "do-gooder" saw that he was virtually at the gates of heaven he would let down his guard or become Shanghaied and become ripe pickings for the demonic hordes. These types of souls would provide plenty of fuel and nourishment for the fires of Hell.
Again, the poets reasoned in the following manner: It is extremely difficult to find the gates of Heaven. It is not so hard to find the Devil. Since the Devil is likely to be waiting in ambush near the gates of heaven, finding the gates of heaven only requires that one heads straight towards one's own strongest Devil. Of course there is the difficulty of eluding his clutches when you are headed straight towards him, but the poets reasoned that it would be worth the attempt and that in addition, great art might be spat out during the throes of battle. If one could weather this centripetal force, then one would be flung through the doors of heaven though a kind of trampolining effect. In addition, one's art would testify on one's behalf of one's original intent. This lead to the terms "disarrangement of the senses" and "intentional vertigo."
3): Spiritual cultures around the world have come up with different terms for dualism, for the active and passive forces which operate in the world. For the Chinese and they are called Yang and Yin. In the Indian subcontinent there are called Rajas and Tamas. The Christian culture would use the terms Heaven and Earth. In each of these sets of opposites the first term would be the light and active force and the second would be the dark and resistant force. This second force would include the ideas of the Devil. But not so well known is the fact that in each of these sets of opposites there is postulated a relationship between them which has its own name, and which forms a third and invisible force. For the Chinese this is called the Tai Ji, or great ultimate limit; in India it is called Sattva, the balanced pure and spiritually even force. Even in Christianity the third force is known specifically as the holy ghost, which is usually termed the love which exists between the father and son; the son in this instance standing for the passive or resistant force. Therefore it is not simply a case of choosing between opposites but rather coming into a balanced relationship between two only seemingly antagonistic forces which create the world. In this instance, the Devil cannot be avoided, resisted or eliminated. Instead, the Devil and all that he stands for needs to be integrated into a balanced whole. This is very similar to the ideas in psychology of C. G. Jung of the integration of the shadow self as part of the job of individuation, which is the goal of Jungian psychoanalysis.
4): Astronauts are known to lose muscle mass in outer space where there is no gravity. Science realizes that bone density and muscle mass are depended upon weight bearing work against resistance. Without resistance no work can be accomplished. In a world without any friction nothing could develop. Therefore the Devil is necessary for any kind of spiritual development. If he did not exist we would have no chance to develop our spiritual musculature at all.
5): Beelzebub is known as Prince of the Devils. He is also known as Lord of the flies, and flies are connected with the elements space and air. As Lord of the flies Beelzebub commands these forces. To human beings, flies are irksome pests who are obstinate and troublesome. In a way, flies are like our own unwanted habits which seem unamenable to our will and which return to plague us even as we shoo them away. Interestingly, in Hatha yoga there is a figure who is one of the founders of the yoga called Matsyendra. Matsyendra is known as Lord of the fishes and king of the yogis. Why is this so? One reason is that the energies of the body are likened to shoals of fishes who travel through the body and who are as difficult to control as the direction of fish in the water. Therefore, he who has mastered control of the subtle energies of the body, or Prana, is called king of the fishes. Clearly an analogy can be drawn between these two perhaps mythical entities; Matsyendra and Beelzebub both represent a mastery over ordinarily uncontrollable forces. One could also deeply ponder the connection between the flies and air, and the fish and water; Air and Water being two of the great elements postulated by ancient esoteric teachings worldwide.