Guidelines for Declaring a Specific Person an Open Sinner or a Disbeliever

Question:
A questioner asks: What are the guidelines for declaring a specific person an open sinner or a disbeliever, and are these guidelines agreed upon, or is there scholarly disagreement regarding them?

Answer:
There is no doubt that this subject is serious, so it is not permissible for anyone to enter into it except one who is qualified with knowledge and has fear of Allah (Glorified and Exalted is He). He must have knowledge by which he distinguishes the truth from falsehood, and he must have fear of Allah, so that he does not have any personal desire, fanaticism, or bias toward anyone except for the truth. Thus, the person's goal in this matter must be the truth. Then, proceeding to declare someone a disbeliever, an innovator, or an open sinner, this is not resorted to except out of dire necessity. It should not be a field for back-and-forth debate in gatherings or casual conversation. Rather, it is resorted to only out of dire necessity, and it is handled by those who are its rightful people: the people of knowledge, the people of scrupulousness, and the people who are free from personal desires and fanaticisms. And the goal of it must be to clarify the truth and refute falsehood, not to attack people or take revenge on them.
The guidelines for declaring someone a disbeliever are known: the known nullifiers of Islam that the scholars have explained. The nullifiers of Islam that the scholars have explained in the books of creed and in the books of Islamic jurisprudence. The jurists established a chapter in jurisprudence called "The Chapter on the Ruling of the Apostate," and they explained therein what the nullifiers of Islam are and what are the causes of apostasy. One should refer to these when there is dire necessity.
However, it is necessary to establish the proof against the one who opposes the truth. One must look at whether he has an excuse or not, whether he is excused by it or not. This is necessary. So the ruling is not pronounced hastily without looking at the person's condition, and whether he is one who is excused by ignorance or one who is not excused by ignorance, and whether the error he fell into is something that is known from the religion by necessity, or is it among the apparent matters, or among the hidden matters?
So these considerations are necessary, and this is a field that no one enters except the specialists among the people of knowledge who are firmly grounded in knowledge, and who are known for their mindfulness of Allah (Glorified and Exalted is He), and who are known for their freedom from personal desire and fanaticism. Yes.