Praise be to Allah.
What is animal rennet?
Before answering this question, it is important to know what rennet is.
"Rennet is a yellowish-white substance ([in a skin vessel] - this phrase appears not to fit here) that is extracted from the stomachs of suckling kids or lambs. When a little of this substance is added to milk, it curdles and becomes cheese. In some Arabic-speaking regions, people call this rennet mujabbinah (cheese-maker), and the stomach (from which the rennet is taken) is called kursh if the animal grazes on grass.”
Is animal rennet halal?
The Islamic ruling concerning rennet is that if it is taken from an animal that has been slaughtered according to Shari'ah, then it is pure (tahir) and can be eaten. This is according to the Hanafis, Malikis, Shafi'is and Hanbalis.
They base this ruling on the verse (interpretation of the meaning): "Forbidden to you for food) are: al-maytatah (dead animals - cattle-beasts not slaughtered)." [al-Maidah 5:3] - the rennet becomes impure by virtue of the animal's death, and it is not possible to remove that impurity from it.
"The ummah has agreed that it is permissible to eat cheese so long as it is not mixed with anything impure, such as adding rennet from a source that is not halal because it was not slaughtered according to shari'ah. This ijma' (scholarly consensus) is the evidence for its permissibility."
This is the opinion which Shaykh al-Islam Ibn Taymiyyah thought most correct in al-Fatawa (21/102), where he said:
"It is more likely that their (the Zoroastrians') cheese is halal , and that the rennet and milk of dead animals is tahir (pure)."
Elsewhere in al-Fatawa (35/154) he said:
"With regard to the cheese made with their (some of the kafir Batini groups') rennet, there are two well-known scholarly opinions, as is the case with the rennet from animals slaughtered by the Zoroastrians and Christians, and rennet from dead animals, of whom it is said that they do not slaughter their animals properly. The schools of Abu Hanifah and Ahmad, according to the other of his two opinions, say that this cheese is halal, because the rennet taken from dead animals is tahir (pure), according to this view, and because the (enzymes in) rennet do not die when the animal dies (so, the concept "impure containers don't cause the contents of the container to become impure by contact" ) applies. The schools of Malik, al-Shafi'i and Ahmad, according to the other of his two opinions, state that this cheese is najis (impure), because the rennet is impure according to this view, as they see the milk and rennet of dead animals as impure. In cases where meat is classified as impure because it is not slaughtered properly, the meat is regarded as being the same as dead meat. Both opinions are based on reports narrated from the Sahabah. The first group states that the Sahabah used to eat the cheese of the Zoroastrians, while the second group states that the Sahabah used to eat what they thought was the cheese of the Christians. With regard to this issue, the follower (ordinary Muslim) must follow an 'alim who advises him to follow either of these two opinions.”
For more about halal and haram food, please see this category: Food and Nourishment.
And Allah knows best.