In comparing and contrasting what we learn about the ancient Sumerian (Mesopotamian), Greek, and Hindu religions as they are represented in the great epics of their respective cultures, we can look at the practice of ritual sacrifice as well as the characteristics of the deities.
In The Epic of Gilgamesh, we read that Enkidu and Gilgamesh cut out the heart of the Bull of Heaven and offer it in sacrifice to the sun god, Shamash. In The Iliad, Agamemnon sacrifices his daughter Iphigenia to regain in the good graces of the goddess Artemis. In The Odyssey, Odysseus sacrifices a number of animals, including a ram and ewe, while in Hades. In The Mahabharata, Emperor Bharata performs the Ashvamedha, an elaborate horse sacrifice reserved for kings and emperors. Animal sacrifice, according to these epics, was a part of all three religions.
While the ancient Sumerian and Greek religions are no longer practiced, Hinduism is still a major world faith and has many different branches, the great majority of which do not perform animal sacrifice.
As for similarities and differences among the deities, the Sumerian (Mesopotamian) gods, as we learn in The Epic of Gilgamesh, are concerned with human welfare to some degree but also exhibit some of the same character flaws as humans. When humans pray for help in dealing with Gilgamesh’s abusive behavior, the Sumerian gods create the wild man Enkidu to befriend Gilgamesh and calm his cruel nature. Yet when Inanna (Ishtar), the goddess of love, falls in lust with Gilgamesh and he spurns her, punishment rains down in the form of the Bull of Heaven. When Enkidu and Gilgamesh kill the bull in order to protect the world, the gods punish them further by inflicting a painful and fatal illness on Enkidu.
Like the Sumerian deities, the Greek gods are quick to punish those who offend them. In The Iliad, Apollo sends a plague to punish the Achaeans (Greek forces) when Agamemnon kidnaps Chryseis, the daughter of a priest of Apollo, as a spoil of war.
Like less-than-ideal parents, the Greek gods also show favoritism. For example, we see in The Iliad that Odysseus is under the special protection of the goddess Athena, and it isn’t because he is an especially moral person. I personally imagine the Sumerian and Greek gods as humans with superpowers, whose egos are usually more important to them than the well-being of humankind.
The deities of Hinduism (the world’s oldest living religion) are far more complex in nature and numerous in number than either the Sumerian or Greek gods. They appear and reappear in new incarnations with different names and are known to present deep philosophical and ethical teachings in the epics. We see this in The Mahabharata (The Bhagavad Gita is part of this very ancient work) and in The Ramayana, India’s two great Sanskrit epics. For example, Krishna, a principal character of The Bhagavad Gita, is the eighth incarnation of Vishnu, sustainer of the universe, also called the Preserver. Vishnu sometimes descends to Earth, taking on various “avataras,” or incarnations, so that he can redress (set straight) the balance between good and evil. In The Bhagavad Gita, Krishna acts as charioteer of the hero Arjuna and instructs him in important metaphysical doctrines.
While the deities of all three cultures have specific roles in the history and management of the world, the cosmos, and the underworld, the gods of ancient Sumeria (Mesopotamia) and those of ancient Greece have more in common with each other than with the deities of Hinduism, a faith that is still practiced by millions.