The one God can be explained in terms of the operating system of the human brain. To explain this, I would like to look at some basic ancient Greek and Roman mythology. These are no longer treated as gods, so I will not offend anyone.
The operating system of the human brain is rooted on the inner self. This is the original center of human consciousness. The inner self is connected to natural instinct and DNA. It is integrated with all things associated with the brain and nervous system.
Below that in operating system hierarchy, are the archetypes of the collective unconscious. I call these personality firmware. These are sort of like neural programs connected to natural human instinct and human nature. They are common to all humans and define us as a species. The lion has different personality firmware and an inner self.
As we move upwards from the depths of the natural operating system, we reach the personal unconscious. This is composed of conscious and subliminal memories, associated with the unique POV of each person. All that each of us encounter in life, is deposited as memory there.Much is forward integrated by the brain into our wisdom of experience.
Above that is the ego, or the center of the conscious mind. The ego is who we see as ourself. It will contain immediate and personal memories some of which only intimate people truly know. I think therefore I am. Above the ego is the persona or mask of the ego. The persona is the mask that ego wears in public. It is shaped by culture, stages of life, fad and the times. This is what strangers will see. It is conscious display by the ego often shaped by culture.
The gods of mythology were connected to the inner self and the archetypes of the collective unconscious; personality firmware. These gods were larger than life and larger than the ego. Each god or goddess became the center of collective human worship.
For examples, the God of War and the Goddess of Love were each projections of specific aspects of the personality firmware. The former was associated with fight-flight and the latter love-mating rituals. When one is in love, the ego is not fully in control. Love works better when you-ego, surrenders to the feelings. Fantasy can appear; most beautiful person in the world, and compulsions and mixed emotions can become active This due to the firmware as personified by the goddess. In times of war, one may not be able to control your fear or your savagery, as the firmware plays out. To the ancients the gods were helping or not helping in the day of battle. Rituals appears to they could animate the needed firmware; have fun in battle. Or the couple may pray and make an offering to the goddess of love, to help renew their passion; trigger the firmware of love.
Polytheism helped the ego stay connected to the operating system, but at the level of the firmware. The King of the Gods, was connected to the inner self, which controls the firmware. But to the ancients, the King and Queen of the Gods were often in a precarious position, due to competition from other sub-gods; Inner self versus firmware
From a psychological health point of view, the inner self controls the firmware and will naturally cycle the firmware to maintain neural energy balance. If we consciously choose to use only particular firmware; serve one of the sub-gods, this may help the ego be more specialized, but it can adversely impact the energy balance of the inner self. The gods may not always hear you, as the inner self makes adjustments in the energy balance.
Centered on the inner self was a healthier place, and made one more balance in terms of human nature. Monotheism was connected to the ego targeting the inner self and repressing the individual firmware of polytheism. Instead the inner self would be allowed to cycle, naturally. However, the older way of polytheism was hard to give up, since specialty allows a particular firmware to develop even further, which is rewarded by the superego of culture. Jack of all trades virus a master of a trade. The master will be paid more. The cultural urge is to go back to polytheism.
The trinity of Catholicism is different from the gods of mythology. Each aspect of the trinity, is not so much a specialty firmware, such as the god of war or the goddess of love, which target specific parts of human nature, rather each aspect of the trinity is designed to be the central theme for all of the firmware. God the father, Son and Holy Spirit are expressions of the inner self, and become part of each of the firmware.
The trinity implies the inner self having evolved into a tri-core processor, away from the mono-core design of the ancient times. Any one of those three cores can integrate the firmware and ego. Using all the three cores at the same time may be a future upgrade.