The Old Gods are a nameless set of gods that were honored by the Children of the Forest and later by the First Men who adopted the ways of the Children. The Old Gods were the first religion ever practiced on the continent of Westeros, but was largely supplanted by the Faith of the Seven brought over by the Andals during the Andal Invasion. The Old Gods differed from traditional religions; there were no holy texts, priests, or structured religious ceremonies. Rather, the Old Gods were more a way of life through which the Children of the Forest connected with mother nature, namely the weirwood forests.
CHILDREN OF THE FOREST
As their name suggests, the Children of the Forest had a special relationship with the weirwood forests. To the Children, these forests were sacred and magical. The weirwood trees were considered to be gods to the Children; supreme spiritual entities with which the Children shared a magical relationship. The Children would carve faces into the weirwood trees, and it is said that after the Children passed on, they could still connect with the forests by seeing through the eyes of the faces carved into the weirwood trees. When the weirwood trees bled their red sap, it was believed to be the tears of the Children of the Forest.
The most powerful of the Children were called greenseers; they were considered to be shamans of the Old Gods. They were skinchangers, possessing the ability to enter the minds of the animals around them and control their actions. They also possessed the magic of greensight — a power of prophetic visions that show what is to come.
THE FIRST MEN & THE ANDALS
When the First Men landed upon Westeros, they adapted the religion of the Children. For thousands of years, the Children and the First Men lived in harmony, worshipping the gods of the weirwood forests and honoring mother nature around them.
However, in 6,000BAL, the Andals invaded Westeros and brought with them a new religion, the Faith of the Seven. The Andals conquered the majority of Westeros, wiping out most of the First Men and Children of the Forest. As such, the Old Gods and the “old way” was largely replaced by the Andals’ new religion, language and way of life.
But, the Andals were never able to conquer the North, and the First Men continued to carry on the “old way,” honoring the Old Gods. This explains why today, most of the Northerners, such as House Stark, still honor the Old Gods and hold sacred the weirwood forests, whereas the rest of Westeros prays to the Faith of the Seven.