Shaykh Muhammad ibn ‘Abd al-Wahhaab (may Allaah have mercy on him) said:
“Know that among the greatest things that can nullify Islaam are ten things:
Associating others in worship of Allaah alone, Who has no partner or associate.
The evidence of that is the aayah (interpretation of the meaning):
“Verily, Allaah forgives not (the sin of) setting up partners (in worship) with Him, but He forgives whom He wills, sins other than that”
This also includes offering sacrifices to other than Allaah, such as to the jinn or at graves.
Whoever regards others as intermediaries between him and Allaah and calls upon them to ask them to intercede for him, is a kaafir according to scholarly consensus.
Whoever does not regard the Mushrikeen [including the Jews and Christians] as kaafirs, or doubts that they are kaafirs, or regards their way as correct, is a kaafir according to scholarly consensus.
After enumerating them, he said (may Allaah have mercy on him):
‘In the case of all these things that nullify Islaam, there is no difference whether a person is joking or is serious or is afraid – except in cases where he is forced to do something. All of them are among the things that are very dangerous and which happen very often. The Muslim has to beware of them and fear them happening to him. We seek refuge with Allaah from the things that earn His wrath and His painful punishment. May Allaah bless Muhammad.’”
[Mu’allafaat al-Shaykh Muhammad ibn ‘Abd al-Wahhaab (212, 213)]