it's an ethnic group and a language.

if you're Yoruba, you're Yoruba, just like if you're black, you're black or if you're Chinee, you're Chinese. so there are Yoruba Christians, Yoruba Muslims and Yoruba ancestral worshippers and Yoruba atheists.

it's only in the Diaspora that the particular forms of ancestor worship traditionally practiced by the Yoruba are called "Yoruba religion," but that term makes little sense in Nigeria.