Is it permissible to eat from the sacrifices of those whose beliefs we do not know?

The animals slaughtered by a Muslim or a person of the Book in a lawful manner are permissible to eat, unless a name other than Allah's was invoked upon them.

As for someone whose belief is unknown, but is presumed to be a Muslim due to living in a Muslim-majority country, or someone living in a Christian/Jewish-majority country and there is no evidence to the contrary, their slaughtered animal is permissible to eat. The proof for this is what Aisha narrated: "A group of people said to the Prophet, peace and blessings be upon him: 'Some people bring us meat, and we do not know whether they mentioned Allah's name over it or not?' He, peace be upon him, said: 'You mention Allah's name and eat.'"

However, regarding someone whose religion is unknown, and it is not certain whether they are a Muslim or a person of the Book, the general rule is to refrain from eating their slaughtered animal until their status is ascertained.