All the prophets sent by Allah (Subhaanahu Wa Ta’aalaa) came with the unique message from Allah (Subhaanahu Wa Ta’aalaa), and that was: There is no God but Allah
The message implied that it is not allowed to follow any other deity except Him in any way.
The first 13 years of the Prophet of Islam, Hadhrat Muhammad (SallAllaho Alaihe WaSallam) in Makkah al Mukarramah are significant when we talk about how important is the concept of Tawheed (Monotheism) in Islam. The Prophet Hadhrat Muhammad (SallAllaho Alaihe WaSallam) preached this very concept from the beginning by the order and grace of Allah (Subhaanahu Wa Ta’aalaa) and invited people not to worship anyone but Allah (Subhaanahu Wa Ta’aalaa), the one and only God of the universe.
Prophet of Islam emphasized the importance of Tawheed from the very childhood, for that there is a Hadeeth from Tirmizi, the Prophet (SallAllaho Alaihe WaSallam) said to Hadhrat Abdullah ibne Abbas: “When you ask, then ask Allah, and when you seek help, then seek help from Allah.” (Tirmizi)
The distrusting Quraish offered Allah (Subhaanahu Wa Ta’aalaa)’s Messenger (SallAllaho Alaihe WaSallam) harmony, majesty, riches, and spouses and other than that from the delights of this life in return for him surrender the call to Tawheed, and asked Him not to speak against their false idols. In any case, Prophet (SallAllaho Alaihe WaSallam) didn’t want that from them, rather continued to proceed with his Da’wah (Call towards Islam) and persevered through the attack alongside his Sahaabah (Companions), until the call to Tawheed was successful following thirteen years, after which Makkah al Mukarramah was free from such bad norms and the idols were annihilated.
The Quran says about Tawheed:
قُلْ هُوَ اللَّهُ أَحَدٌ  اللَّهُ الصَّمَدُ  لَمْ يَلِدْ وَلَمْ يُولَدْ  وَلَمْ يَكُنْ لَهُ كُفُوًا أَحَدٌ [الإخلاص: 1-4]
“Say: ‘(The truth is that) Allah is One. Allah is Besought of all, (needing none). He neither begot anyone, nor was he begotten. And equal to Him has never been any one.” (Al-Ikhlaas 112:1-4)
Also it is stated:
وَمَا أَرْسَلْنَا مِنْ قَبْلِكَ مِنْ رَسُولٍ إِلَّا نُوحِي إِلَيْهِ أَنَّهُ لَا إِلَهَ إِلَّا أَنَا فَاعْبُدُونِ [الأنبياء: 25]
“We did not send before you any messenger but We revealed to him that there is no god but I, so worship Me.” (Al-Ambiyaa 21:25)
At another place, Allah (Subhaanahu Wa Ta’aalaa) said in Quraan:
قُلْ إِنَّمَا أَدْعُو رَبِّي وَلَا أُشْرِكُ بِهِ أَحَدًا [الجن: 20]
“Say: ‘I invoke my Lord, and do not associate anyone with Him’.” (Al-Jinn 72:20)
Allah (Subhaanahu Wa Ta’aalaa) further says:
لَقَدْ أَرْسَلْنَا نُوحًا إِلَى قَوْمِهِ فَقَالَ يَا قَوْمِ اعْبُدُوا اللَّهَ مَا لَكُمْ مِنْ إِلَهٍ غَيْرُهُ إِنِّي أَخَافُ عَلَيْكُمْ عَذَابَ يَوْمٍ عَظِيمٍ [الأعراف: 59]
“Surely We sent NooH to his people. So he said: ‘O my people! Worship Allah. You have no god other than Him. I fear for you the punishment of a great Day.” (Al-A’raaf 7: 59)
Quraan also informs us:
وَلَقَدْ بَعَثْنَا فِي كُلِّ أُمَّةٍ رَسُولًا أَنِ اعْبُدُوا اللَّهَ وَاجْتَنِبُوا الطَّاغُوتَ فَمِنْهُمْ مَنْ هَدَى اللَّهُ وَمِنْهُمْ مَنْ حَقَّتْ عَلَيْهِ الضَّلَالَةُ فَسِيرُوا فِي الْأَرْضِ فَانْظُرُوا كَيْفَ كَانَ عَاقِبَةُ الْمُكَذِّبِينَ [النحل: 36]
“We did raise a messenger among every people, with the message: ‘Worship Allah and stay away from the Rebel (the Satan)’. Then, there were some among them whom Allah guided, and there were others against whom deviation (from the right path) was established. So, travel on earth and see how was the fate of those who rejected (the prophets).” (An-Nahl 16:36)
The Prophet (SallAllaho Alaihe WaSallam) also guides us for Tawheed. Hadhrat Mu’aaz bin Jabal reported that the Prophet Hadhrat Muhammad (SallAllaho Alaihe WaSallam) said to him: “O Mu’aaz! Do you know what is the right of Allah upon His servants?”
He said: “Allah and his Messenger know best.”
The Prophet (SallAllaho Alaihe WaSallam) said: “To worship Him alone and to associate none in worship with Him. And do you know what is their right upon Him?”
He said: “Allah and his Messenger know best.”
The Prophet (SallAllaho Alaihe WaSallam) said: “Not to punish them if they do so.” (Bukhari and Muslim)
Tawheed is one of the main pillars of a Muslim’s life. If a Muslim does not believe in the oneness of Allah and even if he has doubts in any way, and he shares the attributes of Allah (Subhaanahu Wa Ta’aalaa) with any other creatures of this universe, a prophet, a saints or an object, he is not altogether a Muslim, no matter his name is Abdullah, Abdul Wahid, Muhammad bin Abdul Ahad, etc. So this is the key aspect and one has to make it as the base for his Imaan (Belief), along with his other belief that Muhammad SAW is the last prophet sent by Allah (Subhaanahu Wa Ta’aalaa).
Secondly, the purpose behind the creation of this universe is also to worship Allah (Subhaanahu Wa Ta’aalaa) only. As Allah (Subhaanahu Wa Ta’aalaa) says in the Quraan:
وَمَا خَلَقْتُ الْجِنَّ وَالْإِنْسَ إِلَّا لِيَعْبُدُونِ [الذاريات: 56]
“I did not create the Jinns and the human beings except for the purpose that they should worship Me.” (Az-Zaariyaat 51:56)
So Tawheed is the very purpose behind the creation of this universe.
Third and the foremost thing is to negate its opposite that is Shirk (Polytheism) in any way. Before the advent of Islam, there were 360 idols including Laat (لات), Manaat (منات), and Uzza (عزی), in the holy Ka’bah. This was totally against the teachings Allah (Subhaanahu Wa Ta’aalaa) sent through His messengers time to time. The Prophet Hadhrat Muhammad (SallAllaho Alaihe WaSallam) also warned the people about the consequences of committing the hideous sin of Shirk. As Allah (Subhaanahu Wa Ta’aalaa) says in the Quraan:
إِنَّهُ مَنْ يُشْرِكْ بِاللَّهِ فَقَدْ حَرَّمَ اللَّهُ عَلَيْهِ الْجَنَّةَ وَمَأْوَاهُ النَّارُ وَمَا لِلظَّالِمِينَ مِنْ أَنْصَارٍ [المائدة: 72]
“In fact, whoever ascribes any partner to Allah, Allah has prohibited for him the Jannah, and his shelter is the Fire, and there will be no supporters for the unjust. (Al-Maaidah 5:72)
There are more types of Shirk as well. According to the Prophet Hadhrat Muhammad (SallAllaho Alaihe WaSallam), anyone who shows off and tries to tell about his goods just to gain some worldly benefits is called hidden Shirk.
So we can conclude by adding a few more lines that Tawheed is totally opposite of the Shirk and it is the very essence and core thing in Islam. And there is zero-tolerance for such a sensitive issue. Allah (Subhaanahu Wa Ta’aalaa) may forgive all sins of his humans but Shirk is not bearable in any way and in any form whatsoever.
May Allah (Subhaanahu Wa Ta’aalaa) forgive us all and help us being the believers in the oneness of Allah (Subhaanahu Wa Ta’aalaa) till the last day of our lives. Aameen!