Imam al-Nawawi said in al-Majmu’ (9/68):
“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 not halal source because it was not slaughtered according to shari’ah. This ijma’ (scholarly consensus) is the evidence for its permissibility.”
The second view, which is that of Abu Hanifah and is one of two opinions narrated from Imam Ahmad, is that rennet from dead animals or animals that were not slaughtered according to Shari’ah is still tahir (pure).
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).”