The opinion of the Shia’ jurists and the School of Thought of the Ahl al-Bayt (the family of the Noble Prophet of Islam (s)) is that meat that has not been slaughtered according to Islamic laws, the ruling is that it is like regular dead meat (animal flesh or meat that is just found dead on the ground at is not known if it was slaughtered Islamically or not.) Their meat (the People of the Book) is haram (forbidden) and eating their meat is not allowed (according to Islamic law.)
From the point of view of the Shia’ Law (Fiqh) the condition for meat to be sold in a halal (permissible) way in the marketplace, meat shops, and stores (supermarkets) is that first of all: The meat must have come from halal animals (animals that are permissible to eat according to Islamic law). Second of all: This animal meat (that is halal) must have been slaughtered according to Islamic laws. In order to fulfill the proper Islamic laws of slaughtering, the conditions below must be followed:
1) The butcher must be a Muslim, or someone who is ruled as a Muslim; like a Muslim child who has not reached the age of puberty yet. And the slaughtering by a non-Muslim person is according to the ruling of a dead animal (above example). In this case there is no difference between a disbeliever of the Book or a disbeliever who is not a Person of the Book.
2) The head of the animal must be cut with something made from iron.
3) At the same time of cutting the head (slaughter) the front of the body of the animal must be facing in the direction of Qibla (Mecca, Saudi Arabia).
4) At the same time of the slaughter, the butcher must say the Name of God on his or her tongue out loud. 5) After the animal has been slaughtered, it must show a few small bodily movements until it is obvious that it was recently alive and now is killed completely.
6) The animal should not be slaughtered from the back of the head (behind the head.)
If all of these conditions are met during the slaughter of an animal, its meat is halal (allowed/ permissible) and can be lawfully used (eaten).
But the Sunnis (except among them the Hanbalis who are in agreement with the Shia’s) have a different opinion regarding this issue. They say that meat that was slaughtered by the hand of a disbeliever (Kufar) that is NOT a Person of the Book (Christian or Jew) is not halal. And from this viewpoint they do not differ from the Shia’s. But eating the slaughtered meat of the disbelievers who are People of the Book is halal to eat. But a Shia’ can not act according to their school of thought! The responsibility of Shia’s (according to Islamic laws) is to act according to the laws of the Ahl al-Bayt (Family of the Prophet).
 According to the words of Sheikh Bahoee in the book Hurma Zaboyeh Ahl al-Kitab, pg. 59, In respect of the slaughtering of the disbelievers (People of the Book) who are not Jewish, Christian or Majusy, there are no disagreements between the Islamic Scholars whether Shia’ or Sunni, until it reaches the case of the disbelievers who are not People of the Book at all and there is only a disagreement about the slaughtering of the Jews, Christians and Majusiyons.
 Tahreer al-Waseela, answer 2, pg. 621.
 Tahreer al-Waseela, answer 2, pgs. 621-623.
 Of course among them the Hanbalis are in agreement with the Shia’s (in their opinions), Hurma Zaboyeh Ahl al-Kitab, pg. 62.
 Sheikh Bahoee in the book Hurma Zaboyeh Ahl al-Kitab, pgs. 59-63 they say: Most of the Shia’ Scholars like Sheikh Mufeed Muhammad ibn Alna’mon, and….the slaughtered meat of the (People of the Book) is forbidden and there it is not permissible to eat from (their slaughtered meat) whether or not the name of God (Allah) has been mentioned over it or not, and the Hanbalis are in agreement with this. And the Hanafis and the Shofe’ees and the Malikis agree that the slaughtered (meat) of the People of the Book is o.k. even if Allah’s Name is not mentioned on it, and a few Shia’ Scholars have doubts (and differences of opinion) on this issue.