Is there a consensus among scholars regarding specific cases or issues in which it is permissible to declare someone an infidel (kafir) without establishing proof, and is one excused who deliberately avoids having the proof reach him?

Whoever insults Allah Almighty or the Prophet, peace be upon him, is judged to be a disbeliever. The only exception to this is someone who is coerced, as indicated by the Hadith of Ammar ibn Yasir and the Almighty’s saying: {Make no excuses; you have disbelieved after your belief}. Ibn Taymiyyah mentioned that if a Muslim insults Allah, he must be killed by consensus, because he is an apostate disbeliever. Scholars have unanimously agreed that anyone who insults the Prophet, or disparages him, or attributes any deficiency or contempt to him, is an insulter of him, and his ruling is death. Khalil al-Maliki said: "Whoever insults a prophet or an angel, or disparages him, or belittles his right, shall be killed." Aliash mentioned that even if one did not intend insult or contempt, if he utters a word of disbelief by cursing or insulting the Prophet, he is to be killed without delay.