Cursing Muslims is a major sin but is not disbelief
5-1-2006 | IslamWeb
Question:
I have heard a lecture on a tape two months ago about Fiqh and other, one of the sin that this Mufti explained was that if a Muslim curse the mouth of another Muslim (example eat shit) or anything else about the mouth then that person has committed Kuffar. Please explain this situation because I have committed this sin and so does a lot of Muslims.
Answer:
All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
We have already issued Fatwa 86030 on the ruling of cursing, so please refer to it. Cursing in general, using obscene language and insulting people is a sign that the person's belief is not complete, and it is a sign that he is a dissolute person and that he is deficient in his religion. Allaah Says (what means): {And speak good to people [say good words].}[Quran 2:83]. Moreover, it is confirmed that the Prophet 
said: "Indeed the believer is not a slanderer nor a cursor nor a vulgar outrageous person." [Ahmad and At-Tirmithi] It is also confirmed that Ibn Mas'ood 
narrated that the Prophet said: "Swearing (bad mouthing) to a Muslim is a debauchery and an immoral act, and fighting a Muslim is an act of disbelief." [Al-Bukhari and Muslim]
Therefore, swearing, insulting and speaking obscene language is something that subjects the person to the Wrath of Allaah as the Prophet said: "There is nothing that is heavier on the scale of a believer on the Day of Judgement than good conduct, and Allaah hates an obscene and immoral person." [At-Tirmithi] The Prophet further said: "Allaah hates an obscene and vulgar person." [Abu Daawood] The above narrations and many other narrations are evidence that swearing and insulting is forbidden and that this is a great major sin, and this includes swearing and insulting with the tongue and other forms of swearing.
However, we do not know that swearing may take the person out of the fold of Islam, as the Mufti you mentioned in the question said, except if the person who swears and insults believes that this is permissible while knowing that it is forbidden, in this case one becomes a non-Muslim because he is rendering what is forbidden as permissible.
Finally, it should be noted that repenting to Allaah while fulfilling all the conditions of repentance mentioned in Fatwa 87903, wipes out all the previous sins out of Mercy from Allaah.
Allaah Knows best.