The concept of deities in the science of Spirituality is best explained with an analogy. Have you ever wondered why there are myriad colours in the world when we have only three primary colours i.e. red, blue and yellow?
We know that various permutations and combinations of the three primary colours lead to so many new colours.
So also though God is one, He manifests in various ways to perform the various individual functions in the Universe. In order to perform the various functions in the Universe, God manifests in five primary principles. They are:
Creation: Everything is created
Sustenance: Everything that is created is sustained
Dissolution: Everything that is created and sustained is destroyed
Multiplicity: God is in everything
Bliss (Ānand): The quality of God is supreme and everlasting happiness
These five primary principles combine with each other in myriad ways to give millions of unique aspects of God to perform various functions in the Universe. For example, there is an aspect of God that facilitates learning, teaching, protecting, health etc. According to the science of Spirituality each such aspect of God has a form and is known as a deity. There are a total of 330 million deities, each having their unique role in the Universe.