The ancient Greeks possessed a complex religious system, but there was no established creed such as in the Judeo-Christian Scriptures. There was no mystical rapture (except for the mystery cults and the worship of Dionysus) There were importations from Asia. By its first appearance during the Hellenistic period ( 323-146 BC), all the important mythology had already been established as seen in the "Iliad" and "Odyssey." The gods and godesses were divided into those of heaven, earth and sea.
The celestial gods were believed to dwell in the sky on Mount Olympus. The earth gods lived on or under the earth, other gods lived in the sea. The gods were immortal and they controlled the world and nature. But as poweful as they were, their control was limited by Ananka, i.e. necessity
Zeus was that the head of the hierarchy, the father of gods and men. His wife, Hera was the queen of heaven. The gods were fickle with human failings. Since these divine beings controlled the powers of nature, it was important for mere humans to placate and honor them. In general the relationship between the gods and men was friendly, but hubris among men could provoke divine displeasure and cause punishment. Only the gods could be proud, mere humans must know their place in the grand scheme of things. The Greeks had few professional priests, although there were oracles such as those at Delphi. Gererally the Greeks did not see the need for an intermediary between themselves and the gods. There were many alters in Greek homes. Festivals were held regularly, offerings were made to the gods to curry divine favor. There were divine beings for the kitchen, hearth and storeroom. All needed to be worshipped.
The Greeks believed that the soul survived after death, but their ideas about the nature of the afterlife were indefinite. The soul may stay near the tomb or go to a shadowy region. Surviving relatives were obligated to give offerings to deceased family members or risk injury. The natural world was populated by spirits: rivers, lakes, trees and stones had spirits, as did the weather. The Greeks believed that some beings were the offsprings of the major gods, especially Zeus who fathered lesser divine beings such as Hercules. The gods also interceded in human affairs, either to harm or help.
The divine hierarchy was complex. Zeus and Hera were the king and queen. Athena was the goddess of wisdom and war and was an important civic goddness. Apollo was the god of light and music, his sister Artemis was the goddess of wildlife and the moon. Ares was the god of war and Aphrodite the goddess of love. Hermes was the divine messenger. There were many lesser dieties: Helios was the sun god, Iris the goddess of the rainbow. The sea was ruled by Poseidon, a major god. Hades was the god of the underworld.
The myths of the ancient Greeks are fascinating to read. The adventures of Heracles, Theseus and Jason for example are full of adventure. Many of the myths offer insight into the human condition. The stories of heroes and gods have served as inspiration to authors and for artists for many years.
The Romans adopted much of the mythology of the ancient Greeks and their gods. They had their own names for the Greek gods. For example Zeus was the Roman Jupiter, Hera was Juno, Hades was Pluto, Poseidon was Neptune and Athena was Minerva.