Assalaamu alaykum. In islam, the sharia is so wide and Fiqh is so vast, many people joke about and reject normal things and things that they do not like, and they also joke about with many issues, even things that are part of islam, without them realizing it, like mocking the idea of brushing one's teeth (which can lead to mocking using the miswaak [twig or the arak tree used as a tootbrush]), sitting on the ground to eat, or marriage. There are more of such issues which do not come to my mind right now. The point is that people can easily fall into kufr (disbelief) without realizing it. What is the cure for that, and what does Islam say about these matters, evey day matters that people might mock for a joke or reject without even realizing their connection with islam?

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.

In general, it is impermissible to mock any part of the Islamic sharee'ah; whoever does so, even if jokingly, has committed an act of disbelief taking him out of the fold of Islam. Allaah, The Exalted, says (what means): {And if you ask them, they will surely say, "We were only conversing and playing." Say, "Is it Allaah, His verses, and His Messenger that you were mocking?" Make no excuse; you have disbelieved after your belief. If We pardon one faction of you - We will punish another faction because they were criminals.} [Quran 9:65-66] For more benefit, please refer to fatwa 26193.

This is the general religious ruling on such an act. However, when it comes to declaring a specific Muslim individual to be a disbeliever, the relevant conditions must be fulfilled and the impediments must be eliminated, as previously highlighted in fataawa 8106 and 19773. In general, the basic principle is that the Muslim individual is established to be a Muslim, and he is not declared otherwise based on mere doubts; rather his disbelief must be established with certainty.

Moreover, not all the acts believed by common people to be acts of disbelief actually take the doer out of the fold of Islam. Among the examples mentioned in the question is mocking the act of brushing one's teeth; in fact, mocking such an act does not necessarily entail mocking the Sunnah of using the siwaak. In addition, it cannot be said that it is an act of the Sunnah to eat while sitting on the ground and accordingly assume that whoever mocks such an act is ridiculing the religion of Allaah.

Indeed, teaching the Muslims their religion and clarifying the greatness of the Islamic sharee'ah and the grandeur of Allaah, The Exalted, helps fostering the refined status of Islam within their hearts and, accordingly, this will then safeguard them from ridiculing whatever is established as part of the Islamic sharee'ah.

Allaah knows best.