Thinking That A Person Who Reviled the Prophet Is a Kafir
15-9-2022 | IslamWeb
Question:
A person asked about the repentance from reviling the prophet (pbuh) to a sheikh. What is ruling for a person who thinks in his mind this person who reviled the prophet (pbuh) Kafir and then stopped immediately since he knows the issue of takfeer is big since I don't know if all the impediments where met for takfeer. Does that make me Kafir since it might have fallen into doubting the disbelievers to be disbelievers?
Answer:
All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad 
is His slave and Messenger.
It appears from your question and your many previous questions that you clearly suffer from Waswas (obsessive satanic whispering). Therefore, you do not need Fatawa nor jurisprudential research, but rather need to treat the Waswas by disregarding these whisperings and striving to get rid of them by consulting psychological specialists.
What you mentioned does not constitute Kufr (disbelief), but rather mere satanic whisperings. You are still a Muslim, and should not worry about it, and should not get carried away by considering these whisperings.
As for the principle “Whoever denies that a non-Muslim is a disbeliever or doubts his disbelief, is declared a disbeliever.”; it applies to the non-Muslims who are decisively declared disbelievers like the Jews, the Christians, and pagans.
As for a Muslim who has committed any of the nullifiers of Islam (acts that take the doer out of the fold of Islam), it is impermissible for lay Muslims to label him as a disbeliever, not to mention saying that whoever does not declare him a disbeliever is considered a disbeliever! No Muslim individual may be declared a disbeliever unless the conditions of declaring him a disbeliever are met and there are no impediments to declaring him as such. Only the scholars in Sharee‘ah courts who can verify that all the conditions of Takfeer (declaring a Muslim as a disbeliever) are met and its impediments are absent, eliminate all potential misconceptions and doubts, and urge him to repent before judging him to be a disbeliever.
For more benefit on declaring someone as a non-Muslim, please refer to Fatawa 91677, 87963, 91777, 91372 and 133655.
Allah knows best.