The sin of shirk is to associate partners with Allah, basically this means to set up idols and false deities alongside Allah, granting them equal status to Allah, and to give them actions which are only reserved for God alone, such as worshipping them, making oaths to them, having fear and hope in them, and to sacrifice things in their name. All of this constitutes shirk, an example is the case of Jesus, those who set him up as a God, and invoke on him and pray to him are guilty of shirk, they are guilty of setting up a partner to the true God Allah.
In this article we shall cite the forms of shirk that can occur, however so before doing this let us post the verses which show the serious consequences and magnitude of this sin:
004.048 YUSUFALI: 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.
004.116 YUSUFALI: Allah forgiveth not (The sin of) joining other gods with Him; but He forgiveth whom He pleaseth other sins than this: one who joins other gods with Allah, Hath strayed far, far away (from the right).
These verses make it crystal clear that associating partners with God (shirk) will never be forgiven, every other sin you commit, major or minor can be forgiven. Hence a person must do his utmost to avoid this sin because if one dies upon this sin then this shall doom the person leaving no hope for him.
The Prophet said one statement and I said another. The Prophet said "Whoever dies while still invoking anything other than Allah as a rival to Allah, will enter Hell (Fire)." And I said, "Whoever dies without invoking anything as a rival to Allah, will enter Paradise."
So therefore there should be no doubt as to the severity of this sin, again I repeat one must do all he can to avoid this sin as it is the unforgivable sin and shall doom the one who commits it.
Now some may ask how one commits shirk (associate a partner or another god beside the true God Allah). Well the answer to this is that there is more than one way in committing shirk, and it is to this that we now turn our attention to, because it is very important to know the routes that lead to shirk, because when one knows this information it allows him to guard himself against it.
For starters there is a correct way of believing in Allah, one of the correct ways to believe in Allah is that you believe that he is the sole creator, that he is one, that all affairs are under his control and go back to him, that nothing occurs without his will, and that only he can protect or harm you. The verses of the Quran that establish this point are too numerous to quote, but we shall quote a few of these verses to make things easy:
112.001 YUSUFALI: Say: He is Allah, the One and Only;
002.117 YUSUFALI: To Him is due the primal origin of the heavens and the earth: When He decreeth a matter, He saith to it: "Be," and it is.
003.109 YUSUFALI: To Allah belongs all that is in the heavens and on earth: To Him do all questions go back (for decision).
So a Muslim must believe this and not contradict it. Hence one form of shirk is to contradict this belief, it is to believe that some other deity has the power to independently create and cause death. A form of this shirk is committed by Christians, they believe Jesus is the creator of the heavens and the earth, that Jesus is in control of destiny, that Jesus is in control of life and death, therefore this is one type of shirk. So basically believing in any other deity as the source of power which causes life and death, which runs the universe and so on, this is one form of shirk.
A Muslim must be careful from falling into this form of shirk since it possible, there are Muslims who have fallen into this form of shirk by believing that certain saints of theirs can cause life and death, which we all know is not possible since it is only Allah who controls life and death.
Now another correct way of believing in Allah is believing that all forms of worship are for him alone, your prayers, invocations, sacrifices, and oaths are all directed to him. The Quranic verses which speak of this are too numerous to quote, but we shall highlight a few of them to make things easier:
YUSUFALI: Say: "Truly, my prayer and my service of sacrifice, my life and my death, are (all) for Allah, the Cherisher of the Worlds:No partner hath He: this am I commanded, and I am the first of those who bow to His will.Say: "Shall I seek for (my) Cherisher other than Allah, when He is the Cherisher of all things (that exist)? Every soul draws the meed of its acts on none but itself: no bearer of burdens can bear of burdens can bear the burden of another. Your goal in the end is towards Allah: He will tell you the truth of the things wherein ye disputed." (6:162-164)
072.018 YUSUFALI: "And the places of worship are for Allah (alone): So invoke not any one along with Allah;
So to contradict this is to commit shirk, and by contradicting this it means you worship someone else besides Allah, or you sacrifice and send invocations to someone other than Allah. In fact the pagans of the prophet Muhammad's time were guilty of this form of shirk, they set up other idols beside Allah as intercessors so they could supposedly get nearer to Allah, hence they would invoke and call upon these false deities. Most Muslims who fall under this category of shirk make excuses similar to that of the old pagans, that they simply call upon the dead and the saints so they can get nearer to Allah and so on, yet there is no difference at all and Allah rejects this since he has called on you to call on him only and not on others. It does not matter what or who you are worshiping and invocating to, they all fall under the same evil category, whether you are invoking and calling upon the prophet Muhammad, the angels, the jinns, the trees, the saints, Jesus, or false wooden objects you have carved out.
The Quran speaks a lot about this form of shirk, it speaks against it:
"'Those unto whom they pray [i.e., the saints] themselves seek avenue to their Lord's mercy, compete in coming closer to Allah and fear His punishment,' Qur'an
The Quran also refutes those who make excuses, saying we only call upon them to get closer to Allah:
039.003 YUSUFALI: Is it not to Allah that sincere devotion is due? But those who take for protectors other than Allah (say): "We only serve them in order that they may bring us nearer to Allah." Truly Allah will judge between them in that wherein they differ. But Allah guides not such as are false and ungrateful.
So therefore you should be in no doubt concerning who you should worship and who you should devote yourself to.
The third correct way in believing in Allah is believing in his names and attributes. Allah's 99 names are to be found throughout the Quran, and believing in them is also called for in the authentic hadiths. For example one of Allah's 99 names is:
Al-Aleem, which means the All-Knower, that he Knows everything, that he is not ignorant of anything.
So therefore a person who believes another person such as a saint is all-knowing has committed shirk, likewise if they attribute other names and attributes of Allah to themselves, such as being all-powerful, then they have also committed shirk since these names and the meaning and attributes behind the names are exclusively and uniquely for Allah alone.
So in conclusion a Muslim and any non-Muslim must avoid all these forms of shirk, to do so grants you security and safety in the here-after, and to be able to guard yourself from shirk then you must correctly believe in Allah, therefore I implore all Muslims to learn about the correct Islamic aqidah so they will not fall into miss-guidance and error. And I call on non-Muslims to come to the straight path of monotheism since that is indeed the best path.
And Allah the one and true creator whom all worship is due to Knows Best!