The most fundamental article of faith in Islam is belief in strict monotheism (tawhid). The opposite of tawhid is known as shirk, or associating partners with Allah. This is often translated as polytheism.

Shirk is the one unforgivable sin in Islam, if one dies in this state. Associating a partner or others with Allah is a rejection of Islam and takes one outside of the faith. The Quran says:

"Verily, Allah forgives not the sin of setting up partners in worship with Him, but He forgives whom He wills sins other than that. And whoever sets up partners in worship with Allah, has indeed strayed far from the path." (4:116)

Even if people try their best to live a virtuous and generous life, their efforts will count for nothing if they are not built on a foundation of faith:

"If you join others in worship with Allah, then surely all your deeds will be in vain, and you will certainly be among the losers." (39:65)

"Say: 'Call upon other (gods) whom ye fancy, besides Allah. They have no power, not the weight of an atom, in the heavens or on earth: No (sort of) share have they therein, nor is any of them a helper to Allah." (34:22)

"Say: "Do ye see what it is ye invoke besides Allah. Show me what it is they have created on earth, or have they a share in the heavens bring me a book (revealed) before this, or any remnant of knowledge (ye may have), if ye are telling the truth!" (46:4)

The Prophet (ﷺ) said:
"Keep away from the seven fatalities." It was asked: "What are they, O Messenger of Allah?" He (ﷺ) replied, "Associating anything with Allah in worship (i.e., committing an act of Shirk), sorcery, killing of one whom Allah has declared inviolable without a just cause, devouring the property of an orphan, the eating of usury (Riba), fleeing from the battlefield and accusing chaste believing women, who never even think of anything touching their chastity."