The prophet Muhammad PBUH sent by Allah with a Tawheed mission: to ask people to worship Allah alone and there is no other God but Allah.
“Say: I am only a mortal like you. My Lord inspireth in me that your God is only One God. And whoever hopeth for the meeting with his Lord, let him do righteous work, and make none sharer of the worship due unto his Lord.” [Quran 18:110]
Tawheed is the Essence of Prophets’ Teaching
All prophets such as Abraham, Moses, Jesus, and Muhammad teach Tawheed to their people, that is to worship only one God: Allah and not to join Allah with others.
“Lo! Abraham was a nation obedient to Allah, by nature upright, and he was not of the idolaters” [Quran 16:120]
“And afterward We inspired thee (Muhammad, saying): Follow the religion of Abraham, as one by nature upright. He was not of the idolaters.” [Quran 16:123]
Luqman the faithful in the Quran tell his children not to worship other god beside Allah.
“And (remember) when Luqman said unto his son, when he was son! Ascribe no partners unto Allah. Lo! to ascribe partners wrong” [Quran 31:13]
Every parent should follow the example of Luqman to put Tawheed in his children.
Tawheed in 2 Syahadat Sentences
In Islam, testify about the oneness of God is the first of the five pillars of Islam. To enter Islam, people should testify there is no God but Allah and Muhammad is His messenger.
From Ibnu Umar ra, The Prophet PBUH said: Islam has five pillars: Worshipping Allah only, prayer 5 times a day, giving zakat (donation), fasting in the month of Ramadan and doing Hajj (Pilgrimage) [Bukhari-Muslim]
Do not Join Allah with Others
Allah is the Creator of All.
“Lo! I have turned my face toward him Who created the heavens and the earth, as one by nature upright, and I am not of the idolaters.” [Quran 6:79]
Praise be to Allah, Who hath created the heavens and the earth, and hath appointed darkness and light. Yet those who disbelieve ascribe rivals unto their Lord.” [Quran 6:1]
If someone worship other god beside Allah, for example: idols, that is in vain since the idols is not God the Creator of All. In fact idols were made by people.
“Attribute they as partners to Allah those who created naught, but are themselves created” [Quran 7:191]
“Say: Serve ye in place of Allah that which possesseth for you neither hurt nor use? Allah it is Who is the Hearer, the Knower.” [Quran 5:76]
Worshipping Jesus as a god is the greatest and unforgivable sin. God is the Creator of the Universe whilst Jesus is not. Jesus is only a man that was born from his mother’s womb, Mary.
They surely disbelieve who say : Lo! Allah is the Messiah, son of Mary. The Messiah (himself) said : O Children of Israel, worship Allah, my Lord and your Lord. Lo! whoso ascribeth partners unto Allah, for him Allah hath forbidden Paradise. His abode is the Fire. For evildoers there will be no helpers.” [Quran 5:72]
Those who say that God has a son and born like a man so there is more than 1 God like the Father and the Son are nothing but Disbelievers.
How come Allah has a child while He has no wife? Are they thinking that God committed adultery with Mary so has an extra marital child? Allah deny the lie:
The Originator of the heavens and the earth! How can He have a child, when there is for Him no consort, when He created all things and is Aware of all things?” [Quran 6:101]
In the Chapter Al Ikhlas stated:
”Say: He is Allah, the One!
Allah, the eternally Besought of all!
He begetteth not nor was begotten.
And there is none comparable unto Him.” [Quran 112:1-4]
Worshipping other God beside Allah is the greatest sin:
“Turning unto Allah (only), not ascribing partners unto Him; for whoso ascribeth partners unto Allah, it is as if he had fallen from the sky and the birds had snatched him or the wind had blown him to a far off place.” [Quran 22:31]
”Say (O Muhammad, to the disbelievers): Travel in the land, and see the nature of the consequence for those who were before you! Most of them were idolaters.” [Quran 30:42]
The Quran verses above is very clear. Shirik or worshipping many God (Polytheism) is the greatest sin.
Shirik (Polytheism) is the Unforgivable Sin by God.
Shirik or worshipping other beside Allah is the unforgivable sin. This is what Allah said about Shirik in the Holy Quran:
“Lo! Allah forgiveth not that a partner should be ascribed unto Him. He forgiveth all save that to whom He will. Whoso ascribeth partners to Allah, he hath indeed invented a tremendous sin.” [Quran 4:48]
”Lo! Allah pardoneth not that partners should be ascribed unto him. He pardoneth all save that to whom He will. Whoso ascribeth partners unto Allah hath wandered far astray.” [Quran 4:116]
The Polytheists’ Good Deeds are Useless
If someone worship other God beside Allah, then all their good deeds are useless. You may wonder why. But Allah hates so much those people. Imagine if your wife is a very jealous wife, and you take other women as your wives. Though you are good to all of them, yet your wife will hate you and maybe divorce you.
God is the Greatest and nothing compares to Him. But I hope the simple illustration could make you understand this
“If they had set up (for worship) aught beside Him, (all) that they did would have been vain.” [Quran 6:88]
“And verily it hath been revealed unto thee as unto those before thee (saying): If thou ascribe a partner to Allah thy work will fail and thou indeed wilt be among the losers” [Quran 39:65]
”It is not for the idolaters to tend Allah's sanctuaries, bearing witness against themselves of disbelief. As for such, their works are vain and in the Fire they will abide.” [Quran 9:17]
Tawheed, acknowledge there is no god but Allah is the first and the most important thing that should be learnt by a Muslim. Our prophet, Muhammad PBUH, for the first 13 years as a prophet spreading the Tawheed teaching to the Quraish disbelievers in Mecca, and so is after that. All prophets such as Noah, Abraham, Moses, Jesus, and Muhammad teaching the Tawheed to their people.
Without Tawheed, all of our good deeds will be in vain.