"Order is a condition of logical arrangement among the separate elements of a group. It is the structure of social organization. It is the rule of law and custom or the observance of a prescribed procedure. Order was established at the dawn of all creation. Creation's growth was thereafter guided by preordained random elements which were themselves the workings of perfect order. Creation soon gave rise to sentient life forms whose sense of logic and reason were flawed. This event in itself was a result of the progressive workings of perfect order. Soon, however, these sentient life forms began to strive against their predestiny. With each forward step in their development, they followed the inefficient logic of their underdeveloped minds. Thus was the need for intergalactic order-keepers recognized. Thus was the Corps formed."You can read more in the 1989's GREEN LANTERN: EMERALD DAWN, a retelling of Hal Jordan's origin.
"The Corps is the root of all order and knowledge. The Corps is the root of all order and knowledge. The Corps is the maintainer of life and guardian of truth. Without the Corps there would be only chaos and darkness. The Corps is without fear. Without anger or corruption. It is the creation of a highly-evolved mind and maintained by like beings whose destiny it is to maintain the order of things in the universe. The Corps has battled against the forces of chaos for a millennium. To serve is the ultimate honor."
"Knowledge is the sum of all reason and perception. Poser is force prevailing over mass. Power without reason is chaos. Force without knowledge is destructive. The Corps is ultimate knowledge harnessed to ultimate power. The lantern is the source. The Corps is the knowledge. The lantern is the power. Chaos is routed. Order is maintained. Wield the power. Maintain order. It is a great honor."
"The ring and battery are all you shall require. Be sober. Be vigilant. You are one of us now. This is about the survival of the entire races. Of worlds and cultures you've not ever dreamed of. Of a responsibility that transcends your mortal bonds. Of a power greater than your finite mind may imagine."
"I am Green Lantern of space sector 2814. I am dying. You have been chosen to be my successor."
Also recommended, pick up 2009's GREEN LANTERN: SECRET ORIGIN, which is another retelling of the origin. Both are excellent and GL "Must Haves".