The ancient Greek Goddess Demeter was the daughter of the Titans Cronus and Rhea.  Demeter was the goddess of  agriculture, fertility, grain and the earth, sacred to the goddess were agricultural products, livestock, poppy, narcissus and grain.

The Goddess Persephone.

The growth of the harvest every year was associated to Demeter.  It was believed that Demeter taught humanity how to farm through the Greek Hero Triptolemus.  It was said that Demeter sent out the hero with a magic chariot and a bag of seeds with  him.  With the bag of seeds Tirptolemus taught the people how to plant and grow their food.  To honor the goddess the first loaf of bread made from the harvest was sacrificed in her honor.

The ancient Greeks believed that the changing of the seasons was directly related to her.  It was believed that as a result of Demeter's sadness at the absence of her daughter Persephone the goddess would cease to bring life to the world and thus bring upon the earth winter, the return of her daughter would return her to her tasks and thus bring forth the spring, (see Persephone · The beautiful daughter of Demeter, and how she became queen of the underworld).

 
Demeter with her beloved daughter Persephone.

The center of worship towards Demeter was at Eleusis near Athens. At Eleusis the ancient Greeks carried out secret rituals every five years  known as the Eleusinian Mysteries that were based on Demeter's search for her Daughter Persephone. (see Persephone · The beautiful daughter of Demeter, and how she became queen of the underworld). 

Little is known on the Eleusinian rituals as the participants who took  part in these rituals were sworn to secrecy. These rituals were based on the belief that the soul is immortal and that rewards or punishments occurred after the physical death. The central belief in the ceremonies was thought to be that the rebirth of the soul can occur after the physical death in the same manner that the grain can return after its' winter death.