Tawhid (Faith in the Unity of Allah) is divided into three articles:
Believing in the Oneness of Allah in the sense of His being the only Creator, Preserver, Nourisher, etc. This belief is called tawhid ar-rububiyah;
Acknowledging that Allah Alone is the One and Only true God who deserves to be worshipped and thus abstaining from worshipping any other being or thing. This belief is called tawhid al-uluhiyah;
And Having faith and belief in the Oneness of Allah’s Names and Attributes. This belief is called tawhid al-asma was-sifat.
As for shirk (associating anything or anyone in worship with Allah), it is divided into three types:
Major shirk (ash-shirk al-akbar) which Allah does not forgive. Allah says regarding shirk:
“But if they had joined in worship others with Allah, all that they used to do would have been of no benefit to them.” [Al-Qur’an, Surah Al-An’am, 6: 88]
“It is not for the mushrikin (polytheists) to maintain the Mosques of Allah (i.e. to pray and worship Allah therein, to looking after their cleanness and their building, etc.), while they witness against their own-selves of disbelief. The works of such bear no fruit, and in fire shall they dwell.” [Al-Qur’an, Surah At-Tawbah, 9:17]
The one who associate others with Allah and dies on this shirk will not be forgiven and Jannah (Paradise) is forbidden to him, as Allah, Most Mighty and Honored, says:
“Verily, Allah forgives not that partners should be set up with Him in worship, but He forgives anything else, to whom He pleases; and whoever sets up partners with Allah in worship, he has indeed invented a tremendous sin.” [Al-Qur’an, Surah An-Nisa’, 4:48]
“Verily, whosoever sets up partners in worship with Allah, then Allah has forbidden the Jannah for him.” [Al-Qur’an, Surah Al-Ma’idah, 5:72]
Asking the dead or idols for help, slaughtering for them and making vows for them are examples of this shirk.
Minor shirk (ash-shirk al-asghar) which is stated in the Qur’an or in the Prophet’s tradition but is not the same as major shirk. Riya (showing off) and swearing by other than Allah are examples of this kind of shirk. Prophet Muhammad, sallallahu ‘alayhi wa sallam, said:
“Of which I fear for you the most is minor shirk.” When he was asked what was it, he said, “Riya.”
He, sallallahu ‘alayhi wa sallam, also said, “He who swears by anything other than Allah commits minor shirk.“
The Prophet, sallallahu ‘alayhi wa sallam, also warned, “Do not say, ‘Had Allah and such and such (person) willed’, but say, ‘Had Allah then such and such (person) willed.’ “. This kind of shirk does not necessarily lead to disbelief from Islam or an eternal stay in Hell. It negates, however, the completeness of faith.
Hidden shirk. Prophet Muhammad, sallallahu ‘alayhi wa sallam, explained, “Shall I not tell you of which I fear for you more than I fear of the Antichrist?” They said, “Yes, O Messenger of Allah”, and he said, “Hidden shirk, where one beautifies his way of praying only because another one is looking at him.”
Alternatively, shirk could be divided into two kinds – major and minor. In this case, the hidden shirk encompasses both the major and minor kinds depending upon the act committed. It is major if it is the same as the shirk of the hypocrites who hide their false beliefs while showing off Islam out of fear. It is minor if it is the same as riya.