The Plain Indians believed in many gods. They believed that gods showed themselves as the sun, the moon, the stars. They also believed that all animals were controlled by underwater spirits, and an upper world ruled by the thunderbirds.
The two animals they saw as spiritual animals were the eagle and the buffalos.
To most tribes, eagle is referred as thunderbird in stories. It was believed that when eagles open and close their eyes, they cause lightning. They also saw the eagle as God's messenger.
In religious ceremonies, Eagle feathers were rewarded to those who acted of bravery - the Eagle feather warbonnet (see "Clothes" section).
As for the buffalos, they saw them as a gift form the gods as it provided them with food, shelter, clothing and all other basic needs. (So too were bears, owls and coyotes!!!)
The Sun Dance
The Plain Indians have their own religious ceremony - the Sun dance. It is aimed at asking the sun (the god!) for strength and to help their tribe thrive! Each tribe has its own Sun Dance but they usually take place in summer and lasts for 4 days. The performer would have their face painted and others would blow through the eagle bone and play the drums to call on the Thunderbird (the god). For all 4 days (except the last day), they stay there without food nor water...
Numbered according to day into Sun Dance:
Another type of dance was the Ghost Dance. This dance is performed at night. They believed that they could speak to their ancestors and the gods, and that doing the Ghost Dance would help get their land back.
Copy and paste the above link to see it in action