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, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
Refer to the books below that consider the fundamentals of Islam, some or all of which are available in English:
- Al-‘Aqeedah At-Tahaawiyyah and its explanation by Ibn Abi Al-‘Izz Al-Hanafi ;
- Fat'h Al-Majeed Sharh Kitaab At-Tawheed by Shaykh Abd-Ar-Rahmaan Ibn Hasan Ibn Muhammad Ibn Abd-Al-Wahhaab ;
- Al-Eemaan by Ibn Taymiyyah ; and
- Al-Aqeedah Al-Waasitiyyah by Ibn Taymiyyah .
Also the famous Hadeeth of Jibreel (Gabriel ) included some of the fundamentals:
- When Jibreel asked the Prophet, sallallaahu ‘alayhi wa sallam , what is Islam, the Messenger of Allaah, sallallaahu ‘alayhi wa sallam, replied: "Islam is to testify that none is worthy of worship but Allaah and that Muhammad is His Messenger; to perform prayers; to pay compulsory charity (Zakah); to observe fasting during the month of Ramadan; and to perform pilgrimage if you can find a way thereto."
- When Jibreel asked the Prophet, sallallaahu ‘alayhi wa sallam, what is Eemaan (faith), he replied: "Eemaan is to believe in Allaah, His angels, His Books, His messengers, the Last Day and predestination – both good and evil."
- When Jibreel asked about Ihsaan he replied that Ihsaan is to worship Allaah as if you see Him, since indeed Allaah sees you even though you do not see Him.
- When Jibreel asked the Prophet, sallallaahu ‘alayhi wa sallam, about the date of the Day of Judgment, the Messenger of Allaah, sallallaahu ‘alayhi wa sallam, replied that, like him, he knew nothing about it. Then Jibreel asked him about the signs of the Hour and the Prophet, sallallaahu ‘alayhi wa sallam, replied to him that from its signs are when a slave woman gives birth to her master and when shepherds become the masters of people. [Muslim]
Imaam At-Tahaawi, a Hanafi scholar further mentioned that Eemaan is to believe in Allaah, His angels, His books, His messengers, the Last Day and that predestination – whether good or evil – is from Allaah The Almighty. Ibn Abi Al-‘Izz, the Hanafi scholar mentioned that these are the foundations of the religion.
Polytheism is a very dangerous thing and should be avoided because the Prophet, sallallaahu ‘alayhi wa sallam, said that the right of Allaah The Almighty over His servants is for them to worship Him without associating any partner with Him - and their right over Him is that He does not punish anyone who worships Him without associating a partner with Him. [Al-Bukhari and Muslim]
It was also narrated that the Prophet, sallallaahu ‘alayhi wa sallam, threatened that whoever dies while worshiping other than Allaah The Almighty will enter Hell. [Al-Bukhari]
Imaam Ibn Al-Qayyim mentioned that Allaah The Exalted does not forgive Shirk, (i.e. associating partners with Allaah The Almighty in worship), regardless of whether it is a human or a stone that is invoked, feared or loved as Allaah The Almighty should be. Therefore, a person must disbelieve in At-Taghoot (any entity worshipped rather than Allaah The Almighty) along with the testimony of oneness, which is to profess that there is no god truly worthy of worship except Allaah.
It was also narrated that the Prophet, sallallaahu ‘alayhi wa sallam, said that if a person professes that there is no god truly worthy of worship except Allaah The Almighty and rejects everything that others worship, his property and blood will become inviolable. He will be judged by his overt beliefs, although Allaah will call him to account for his covert beliefs. [Muslim]
According to this Hadeeth, Shaykh Abd-Ar-Rahmaan Ibn Hasan Aal Ash-Shaykh proved that the property and blood of any person cannot be considered inviolable except after professing that there is no god truly worthy of worship except Allaah The Almighty and rejecting everything that others worship besides Allaah. Consequently, if a person professes this testimony of oneness without disbelieving all the things that others worship, then his property and blood are violable, because he did not reject nor disbelieve in polytheism, as required by the testimony of oneness.
The fundamentals of religion, its nullifiers, conditions and other such issues require more than what we can mention here, however this may be a suitable introduction for acquiring more knowledge on the matter.
Allaah Knows best.