What is the ruling on declaring someone who openly commits a sin and shows no regard for religion to be a disbeliever (kafir)? Is it obligatory to establish proof against him before declaring him a disbeliever, or is he immediately declared a disbeliever without establishing proof?

According to the Sunnis, a sin does not necessitate disbelief unless the perpetrator deems it permissible. This is supported by the hadith: "Verily, as long as you invoke Me and hope in Me, I will forgive you," and the statement of Ibn Abi Zayd: "No Muslim is declared a disbeliever due to a sin." Adultery and drinking alcohol are two major sins that must be avoided. Allah has condemned alcohol in the Quran, and in the Hadith, promised the drinker that his prayer would not be accepted for forty mornings, and in another narration, that he would be given to drink from "tinat al-khabal" (the mud of corruption) or "the pus of the people of Hell." Allah has also condemned adultery in the Quran, and promised its perpetrator a multiplied punishment and eternal humiliation therein. In the Hadith, the Prophet (peace be upon him) saw in his vision those who were being tormented in the oven, and they were the male and female adulterers, because "for a man to be stabbed in the head with an iron needle is better for him than to touch a woman who is not lawful for him." As for the person suffering from obsessive thoughts (waswas), he must distance himself from these thoughts and preoccupy himself with other matters, because declaring a non-disbeliever to be a disbeliever does not lead one out of the fold of Islam, especially in the case of someone suffering from waswas. Ibn Abd al-Barr clarified in his commentary on the hadith "Whoever calls his brother, 'O disbeliever,' then it has returned upon one of them," that the hadith refers to the Khawarij who declare believers to be disbelievers due to sins, and not to someone who says it out of the grave enormity of the sin.