In the name of Allah, Most Gracious, Most Merciful.
Allah writes in his book, the Quran:
And We have indeed made the Qur’an easy to understand and remember: then is there any that will receive admonition? (Surah 54:17)
According to the Quran, the greatest sin of all creation in the sight of Allah is shirk. The meaning of shirk is to stand partners with Allah and his qualities.
Allah says in Surah Al-Nisa:
Allah forgiveth not that partners should be set up with Him; but He forgiveth anything else, to whom He pleaseth; to set up partners with Allah is to devise a sin Most heinous indeed. (Surah 4:48)
In another part in the Quran, the Prophet Luqman, said addressing his son:
Behold, Luqman said to his son by way of instruction: “O my son! join not in worship (others) with Allah: for false worship is indeed the highest wrong-doing.” (Surah 31:13)
Shirk is such a big sin, that even if any Prophet who has earned many blessings (swab) and has remained sinless in his entire life, commits shirk, their lifetime of good deeds would go to waste.
Allah says in the Quran, addressing Prophet Muhammad Peace be upon him:
But it has already been revealed to thee,- as it was to those before thee,- “If thou wert to join (gods with Allah), truly fruitless will be thy work (in life), and thou wilt surely be in the ranks of those who lose (all spiritual good)” (Surah 39:65)
In another place in the Quran, Allah, after talking about 18 of the greatest of Messengers says:
This is the guidance of Allah: He giveth that guidance to whom He pleaseth, of His worshippers. If they were to join other gods with Him, all that they did would be vain for them. (Surah 6:88)
The foundation of our religion is based on the fact that only and only Allah is the Lord of the Universe. Neither anyone is like him, nor any one is partners with him. Every grain, particle, atom is in Allah’s control. Tahueed is the oneness of Allah, and anything other than that is Shirk!