The Faith of the Old Gods is a fictional religion appearing in the A Song of Ice and Fire book series by George R.R. Martin, and in its television adaption Game of Thrones.
Worship of the Old Gods is the oldest known religion in Westeros. It was originally the religion of the Children of the Forest, who practiced it millennia before the coming of the First Men. The First Men initially made war with the Children, but after peace was struck, the First Men eventually came to take the Old Gods of the Children as their own. The Old Gods were the dominant religion in Westeros until the coming of the Andals, who brought their own religion, the Faith of the Seven with them, which would eventually supplant the Old Gods as the majority religion in Westeros, the North being the only region wherein the majority still keep the Old Gods.
Worship of the Old Gods is an animistic religion. The religion teaches that the Old Gods are nameless, faceless deities that live in the stone, earth and trees. There are no clergymen, holy texts, songs of worship, or practically any rites at all that go with the worship of the Old Gods (though it is possible that human sacrifice played a role in the religion during ancient times). Weirwood trees with faces carved in them are considered sacred, and prayers, oaths, and marriage vows are often spoken in the presence of the trees.
Incest, kinslaying and slavery are considered highly offensive to the gods, while hospitality and the laws that surround it are considered sacred.