Tawhid means believing that Allah SWT is the one and only God. It is sometimes referred to as monotheism, however in the purest sense in that no other entity can in any way have the attributes or be equivalent or even in competition with Allah.
All acts of worship that a person does should be to Allah or for Allah Alone. For example, prayer should only be to Allah, religious animal sacrifice should only be done in Allah's name, and fighting to establish and protect religion [Jihad]. Tawhid is therefore the central concept upon which all of the deen of Islam rests.
A. Importance and Benefits of Tawhid.
1. Tawhid Is The Essence Of Islam And The Foundation Of Its Strength.
All other related laws, beliefs and commands of the teachings of Islam must stand firm on this basic foundation. Without it, Islam becomes void and futile. Muslims must adhere to the basic principles of Tawhid in order to be categorized as a true believer of Islam.
2. Tawhid Is The Call Of All The Prophets To Their People.
Allah SWT says:
"And verily We have sent among every nation a Messenger proclaiming Worship Allah Alone and avoid all false deities." [An-Nahl, 16:36]
3. Tawhid Is The Only Reason For The World's Creation.
"And I created not the Jinns and Men except that they should worship Me." [Adh-Dhariyat, 51:56].
By declaring sincere belief in Tawheed, a disbeliever will become a Muslim. Conversely, a Muslim will become a disbeliever [kafir] if he rejects any aspect of it.
4. Through Tawhid We Would Achieve Salvation From Eternal Punishment In The Hereafter, Right Guidance In This World, And Forgiveness For Sins.
Allah states in the Qur'an: "Verily Allah does not forgive associating partners with Him [syirik] however; He forgives all other sins as He pleases." [An-Nisa', 4:48].
The Prophet Muhammad SAW said:
'The worshipper's right on Allah is that He will not punish those who worship none besides Him.' [Bukhari and Muslim]
5. Whoever Fulfils Tawhid Will Be Admitted To The Garden [Al-Jannah] In The Hereafter.
The Prophet Muhammad SAW has stated:
'Verily Allah has promised that whosoever does not commit syirik shall enter Al-Jannah [the Garden in the Heaven].' [Al-Bukhari]
6. It Is The Reason For Solving People's Anxiety.
The true happiness will be achieved if persons realise Tawhid within their heart. They will also never feel psychologically enslaved even if physically captive.
7. It Is The Only Reason For Allah's Pleasure And Rewards.
Allah says: "If you reject faith then know that Allah is in no need of you and He does not accept disbelief from His slaves." [Al-Zumar, 39:7].
Disbelief is nothing but a manifestation of syirik which is the opposite of Tawhid.
8. The Acceptance Of All Of One's Religious Deeds Is Based Upon Tawhid.
Allah states: "Whoever accepts a religion other than Islam it shall be rejected by Allah and in the Hereafter he will be among the losers." [Ali 'Imran, 3:85].
Islam is nothing if not an embodiment of the belief in Tawhid.
9. Tawhid Prevents Man From Eternally Remaining In The Hellfire.
The Prophet Muhammad SAW stated in an authentic hadith:
'Whoever dies and has so much as mustard seed of faith in his heart shall enter Al-Jannah.'
B. Sources of Islamic 'Aqidah [Belief in Monotheism]
The Qur'an is a Divine Revelation which Allah SWT sent down through angel Gibriel to His Messenger Muhammad SAW and has been passed forth to us both in form and in content in an uninterrupted transmission. It is the last scripture to be revealed to mankind. As the actual word of Allah in its original form in the Arabic language it is the first and foremost source of Islamic 'Aqidah.
Allah SWT says:
"And we did not send any Messenger before you but we inspired him [saying] none has the right to be worshipped but Me [Allah] so worship Me." [Al-Anbiya', 21:25]
Hadith in Arabic has the general meaning of speech whether pertaining to religion or not. Then it took a special sense [Prophetic traditions], comprising all the Prophet Muhammad's SAW speech, deeds, or tacit decisions.
In this regard, the term Hadith and Sunnah are synonymous. The Prophet Muhammad SAW, being the fountain from which both the Qur'an and the Sunnah spring, was the best to explain what each and every person should believe about Allah.
The Prophet SAW stated about himself: 'I am the most knowledgeable of Allah amongst you and the most fearful of him.' And he also stated: 'Every child is born on Al-Fitrah [true faith of Islamic Monotheism] but his parents convert him to Judaism, Christianity or Magianism.'
See: The Categories Of Tawhid #7