These are some dangerous beliefs, sayings and actions that take one out of the fold of Islam.
1) Shirk (Associating partners with Allah, polytheism, idolatory) Allah (swt) said: "...Allah does not forgive that partners should be set up with him in worship; but He forgives anything else to whom He pleases..." [4:48] 2) Setting up intermediaries between oneself and Allah, making supplication (du'a) to them, asking them for their intercession (Shafa'ah) and trusting upon them is disbelief by the consensus of the scholars.
3) Anyone who does not consider those who commit shirk to be disbelievers, or who has doubts concerning their disbelief, or considers their way and doctrines to be correct is himself a disbeliever.
4) Anyone who believes that any guidance other than the Prophet's(pbuh) is better, or that any rule and legal decision other than the Prophet(pbuh) is better than his.
5) Anyone who hates and detests anything of what the Messenger of Allah has brought has left the fold of Islam, even if he practices it. As Allah (swt) said: This is because they hate what Allah has sent down, so He made their deeds fruitless." [47:9]
6) Anyone who ridicules or mocks any aspect of the religion brought by the Messenger of Allah or any of its rewards and punishments becomes a disbeliever. The proof of this is in the saying of Allah(swt): " Say: Was it Allah and His signs and His Messenger that you were mocking? Make no excuse; you have disbelieved after your belief." [9:65-66]
7) Practicing sorcery and black magic, or being pleased by it, is disbelief. The proof of this is in the saying of Allah(swt): " But neither of these two (angels, Haroot and Maroot) taught anyone (sorcery and magic) till they had said (warning them): 'Indeed, we are a trial, so do not disbelieve." [2:102]
8) Supporting and aiding the disbelievers against the Muslims is disbelief. Allah(swt) said: "Whoever among you takes them as allies is surely one of them. Truly, Allah does not guide the wrongdoers." [5:51]
9) Anyone who believes that some people are excused from following the Islamic law is a disbeliever, as Allah(swt) said: "And whoever seeks a religion other than Islam, it will not be accepted from him and in the Hereafter he will be from among the losers." [3:85]
10) Avoiding and turning completely away from the religion of Allah by abstaining from learning its precepts or not acting upon it. Allah(swt) said: "And who does greater wrong than he who is reminded of the revelations of his Lord and turns aside from them; truly We shall exact retribution from the guilty." [32:22]