Deity is a quality which controls and governs the actions and thoughts of every living being. There is a beautiful story which describes this in the Kenopanishad.
The devas had a fight with the demons and the devas emerged victorious. In their victory they believed themselves to be all powerful and enlightened. In the process of their jubilation, they forgot the supreme Brahman of which they were a part. So, Brahman came to enlighten them in the guise of a yaksha (spirit).
The devas sent Agni, the god of fire, to investigate who this spirit was, from where he had come and what he wanted. The spirit said, 'I am nothing. Please first introduce yourself. I wish to know who you are, full of your own splendor.' Agni boasted, 'My name is Agni. I am all powerful. I have the strength to burn up the whole creation, the whole universe'.
So, the spirit placed a bit of straw in front of Agni and said, 'Burn it!' Agni replied, 'Oh, that is child's play for me... poof! 'But the straw did not burn. He utilized all of Ms strength and depleted himself completely trying to burn that piece of straw, but he was not able to do it. So, he went back defeated.
The devas then sent another deity, Vayu- the god of wind, and the same thing happened. The wind was unable to move that piece of straw from its place. In the same way, one by one, all the different devas came and tried out their strength against the spirit, and they were all defeated.
Finally Indra, the lord of the devas, decided, to come, but as soon as he appeared in front of the yaksha, the spirit disappeared. Indra was completely bewildered. 'What has happened?' he thought, 'Where is the spirit? Who was he?' At that moment, Uma, the consort of Lord Shiva, appeared and told Indra that the yaksha was not an individual spirit, but the manifestation of supreme consciousness. Then Indra realized his folly and went back to tell the others.
This story represents the relationship of the devas, the dormant human qualities and potentials, with the supreme consciousness. Agni, who came first to enquire, represents vitality, action, movement. When it comes face to face with the supreme consciousness, it cannot match its strength. It is bound by a form, a manifestation, an action, whereas the supreme consciousness is beyond form, beyond manifestation and beyond action. Next Vayu came, which represents the mind. We know that air is never still. Of course, we can close the room and shut out the draft, but there will still be some movement of air. The electrons, the protons, the neutrons, the atoms are constantly in movement, and they are in the air. So, the totality of mind is represented by the element air or wind. Of course, this refers to the gross mind, not the supreme mind.
This gross mind itself cannot meet the strength of the supreme consciousness. It is like all the other qualities which are bound, because devata is a bound quality. All these bound qualities, due to their individuality and grossness, can never match the unbound or supreme consciousness.
Then Indra, the lord of the devas, comes. Indra represents the individual identity, 'I am'. But even though he is highest amongst the devas, he too cannot meet with the supreme consciousness race to face, because this identity of 'I am' singles oneself out. Individual identity acts as a veil and prevents us from seeing the reality which is the supreme consciousness.
Therefore, Uma comes in order to explain the situation, because, being the consort of Lord Shiva, she represents the link between the individual consciousness and the supreme. Suddenly there arises a question in the mind of the individual: 'Who am I ?' With this question, the process of gyana yoga begins. As you become more involved in this process of self-understanding and analysis, and you reach various states of perfection in your life, in your actions, in your behaviour, in your personality, you finally meet the daughter of Himavan, enlightenment!
It is through this process, when you lose yourself, when you try to merge yourself completely with the higher consciousness, that you drop your shade of individuality. Through this process you realize, 'I am not this Individuality; I am nothing but the supreme self. I am That! I am not this finite self; I am that infinite self!' You see the beauty of it. This is not a direct meeting of the individual consciousness with the supreme consciousness. It is just a simple realization, 'Oh, so, I am That!' It is a very beautiful and a very simple realization.