In islam takfirism means appostasy is forbidden, that anyone can declare you an appostate for any reason (usually because you are not an agent useful idiot of the caliph) even if you are muslim, and even a Christian becoming atheist is executed for apostasy. Some even argue that new muslims who converted from Christianity are not even trusted and considered dhimmi appostates too.
Heck, they even kill each other for being the wrong kind of Muslim.