What is the essence of gods unity? What is tawhid?
All divine religions teach the oneness of God. There is nothing else like Him. His Excellency Abraham explained the oneness of God to his father Azer. The fundamental principle that Judaism emphasizes is the oneness of God. According to the Torah, Adam the first man, his children, Noah, Abraham, Isaac, Jacob, and Joseph all invited people to the One God. In the Ten Commandments that were given to HE Moses and elsewhere in Torah, the most emphasized issue is the oneness of God. The Psalms that were revealed to HE David are prayers to One God. His Excellency Jesus also emphasized that the first command in religion is the oneness of God.
Excessive comparisons in Judaism caused an anthropomorphic description of God, and excessive love transformed the human Jesus into God, entailing corruption of the unity of God into a trinity. Islam once again made clear the oneness of God, dispelling the false notions that had obscured it over time, and it invited Judaism and Christianity to join it in affirming the oneness of God (tawhid).
Rational and cosmic evidence of the Creator shows that He is one. Quran says:
“No son did Allah beget, nor is there any god along with Him: (if there were many gods), behold, each god would have taken away what he had created, and some would have lorded it over others! Glory to Allah. (He is free) from the (sort of) things they attribute to Him! (Al Muminun (The Believers), 23:91)
“If there were, in the heavens and the earth, other gods besides Allah, there would have been confusion in both! but glory to Allah, the Lord of the Throne: (High is He) above what they attribute to Him! (Al Anbiya (The Prophets), 21:22)
Since the existence of more than one God implies characteristics of imperfection such as impotence, deficiency, and being created, the Creator’s oneness is a must.
His oneness is also understood from this: the rain that falls from the sky, tomatoes or pepper that grow from the earth, apples and pears that grow in a tree are all the same shapes and colors in the world. Regardless of the distance between them, they act like they know each other. This shows human beings that all existence is created by the same hand.
According to Islam the biggest sins include not acknowledging God, associating partners in His Personage, Attributes, or Acts, and daring to attribute divinity to anything other than Him. This sin is called “polytheism: associating partners” and is the greatest of all great sins. God Almighty characterized polytheism as “the highest wrong-doing” and “to devise a sin Most heinous indeed.”
Allah said that even though He would forgive any of the other sins as He wishes, He would not forgive those who commit polytheism and die unrepentant. The only way out of the sin of polytheism is to leave it and embrace tawhid.
 The Noble Quran, Mariam (Mary), 19: 42-47.
 Genesis, 1:26-28; 4:26; 6:9.
 Exodus, 20:2-3; Deuteronomy, 6:4-5.
 Mark, 12:28-29.
 Âli Imrân (The Family of Imran), 3: 64. Prof. Dr. Ö. F. Harman, Article: “Islam”, Diyanet Islâm Ansiklopedisi Islamic Encyclopedia of (Republic of Turkey, Presidency of Religous Affairs), Istanbul 2001, XXIII, 4.
 Luqmân, 31: 13; An-Nisâ (Women), 4: 48.
 An-Nisâ (Women), 4: 48, 116.
Source: Dr. Murat Kaya,The Final Divine Religion: ISLAM