Bismillah Ar-Rahmaan Ar-Raheem
One of Allah’s attributes is ((الوحدانية))—The only One
This is the most noble of Allah’s attributes, as it attributes to Allah complete oneness and complete authority and power. And that He is the only One reserves the right to be worshipped by every single thing/object in the whole universe and every other place that is beyond our limited comprehension. It is what every single Prophet and Messenger preaches and the base of every Prophetic Revelation. Every single Prophet and Messenger sent to mankind preach the Oneness f Allah, even though the Syariah(Islamic Laws)of the Messengers differ. Allah’s Mercy to Mankind spent 13 years in Makkah preaching the Oneness of Allah. Entire civilisations were destroyed as they refuse to declare monotheism.
For example Allah says in the Quran:
Indeed, We sent Noah to his people, [saying], “Warn your people before there comes to them a painful punishment.” He said, “O my people, indeed I am to you a clear warner, [Saying], ‘Worship Allah , fear Him and obey me.(71:1-3)
The whole of surah 71(Surah Nuh) talks about the people who refuses to declare monotheism, what they do when asked to do so and the reward if they have done so. It also tells us about the severe punishment that befell them for their refusal to declare monotheism.
If Allah has dedicated a whole surah to tell us about the people who refused to declare His Oneness, it must be very very important.
And to Banu Israaeel:
((And [recall] when We took the covenant from the Children of Israel, [enjoining upon them], “Do not worship except Allah…))2:83
And Prophet Ya’quub AlayhisSalaam:
“Or were you witnesses when death approached Ya’quub(Jacob), when he said to his sons, “What will you worship after me?” They said, “We will worship your God and the God of your fathers, Ibrahim(Abraham) and Ismai’l(Ishmael) and Ishaaq(Isaac )- one God. And we are Muslims [in submission] to Him.”2:133
And much much more…
From the attention(At-tanbiih) given to it in the Quran and Sunnah, it is derived from this noble attribute a knowledge called At-Tawheed. As without declaring the Oneness of Allah, every single good deed you do, every other thing you attribute to Allah is useless as the essence of becoming a Muslim is the Syahaadah and the first sentence of the Shahaadah is “أشهد أن لا إله إلا الله”—I bear witness that there is no God but Allah ie. Declaring the Oneness of Allah.