Allah, the Wise, has said:
لاَ تُشْرِکْ بِاللٌّهِ إِِنَّ الشِّرْکَ لَظُلمٌ عَظِيمٌ
"Do not associate aught with Allah; most surely polytheism is a grievous iniquity." 
Imam Baqir (a.s) said:
أََلْمَعاَصِيَ الَّتِــي يَرتَکِبُونَ فَهِيَ شِرْکُ طاَعَةٍ أََطاَعُوا فِيهاَ الشَّيطاَنَ
"The commitment of sins on the part of people is (in reality) polytheism with respect to obedience (of Allah) - in which they obey the Satan." 
One of the vices of the soul is polytheism in which a person, as a result of ignorance, poverty, scepticism and the like, attributes "Allah-ship" to certain things that do not possess the merit and the ability to be Allah; or regards someone, other than Allah, to be inherently and independently efficacious (in affairs); invokes someone, other than Allah, during worship; or intermingles some other (non-divine) intention and objective with his acts of worship.
A polytheist's act of resorting to someone other than Allah is a sin; increase in polytheistic beliefs is brought about by the deceptions of the Satan and persistence of such views lead to hypocrisy and the nullification of deeds, which in turn bring about damnation in this world and the hereafter.
1 – The Extremist Who Considered Himself a Shia
Sahl Ibn ZiyadAdmi relates: "Some of our friends wrote a letter to Imam Askari (a.s) saying: "Ali Ibn Hasakah claims to be one of your friends and followers, but is of the belief that you are Allah and he is the 'door' that leads to you and the Noble Prophet (s.a.w). He is of the opinion that prayers, fasts, zakat and Hajj are nothing but your marifat and according to him, whoever holds this belief is a true and perfect Mu'min and is then relieved of the responsibility of performing the other acts like prayers and fasts."
In reply, the Imam (a.s) wrote: "Ali Ibn Hasakah lies. May the curse of Allah be upon him! I do not consider him to be of my friends. By Allah! Muhammad (s.a.w) and the prophets before him had been sent to preach monotheism and invite the people towards prayers, fasts, zakat, Hajj and wilayah. Never did Muhammad (s.a.w) ever invite anyone towards polytheism, and all of us are the successors of the Noble Prophet (s.a.w) and the servants of Allah, and never do we ascribe partners to Him. If you happen to see one of them, smash his head by means of a stone (due to his polytheistic views).""
Ali Ibn Hasakah was of the Ghulat (The Extreme Shiites), who possessed deviated beliefs and had nurtured students such as Qasim Sharani, Yaqtini, Ibn Baba and Muhammad Ibn Musa Sharifi. The Imam (a.s) his rejection of their polytheistic beliefs when he announced: "I absolve myself from them; May Allah curse them!"
2 – A Polytheist Turns Into a Believer
Shabih Ibn Uthman was a polytheist whose father and brother had been killed by the Muslims in the battle of Uhud. He awaited an opportunity to kill the Noble Prophet (s.a.w) and thus avenge the death of his father and brother.
Years passed and in the year 8 ah the battle of Hunain loomed. During that crisis, Shabih said to himself: "This is an excellent opportunity," and readied himself for battle. In the battle he advanced forward and positioned himself behind the Noble Prophet (s.a.w) in order to execute his sinister intention.
Allah informed the Noble Prophet (s.a.w) of his evil designs whereupon the Noble Prophet (s.a.w) immediately turned around and striking Shabih's chest with his fist, said: "O' Shabih! I seek shelter in Allah from your evils."
Shabih relates: "A shiver ran through my body. I looked at the face of the Noble Prophet (s.a.w) and at that very moment I felt as if he was the most beloved of all persons to me, to the extent that I even perceived him to be dearer to me than my very own eyes and ears. At that very moment I bore witness to the Unity of Allah and the Prophethood of the Noble Prophet (s.a.w) and became a Muslim, after which, I said to him (s.a.w): "I bear witness that Allah informed you of my concealed intentions."'
The Noble Prophet (s.a.w), placing his hand upon my chest, prayed: "O' Lord! Ward the Satan away from him."
After the battle had concluded, he (s.a.w) said to me: "What Allah had desired for you is better than what you had desired for yourself.""
3 – Concealed Polytheism
Abu Said al-Khudri narrates: "We were a few individuals, who, in a difficult and dangerous period, had shouldered the responsibility of guarding the Noble Prophet (s.a.w) in turn and according to a pre-determined schedule. After a period, some of us had become accustomed to speaking to each other softly and in whispers, including myself. One night the Noble Prophet (s.a.w) approached us and, noticing some of us talking secretly and in whispers, said to us: "What is this secret conversation? Have you not been prohibited from this act? (when you confer together in private, do not give to each other counsel of sin and revolt and disobedience to the Messenger).""
We pleaded: "We seek the forgiveness of Allah and His Prophet, (however), we were discussing about Dajjal."
The Noble Prophet (s.a.w) said: "Should I inform you of one, whom I regard to be more dangerous than Dajjal?" He then said: "Concealed polytheism - that is, a person becomes a cause for the sins and evils of others. Danger from such a person is greater than that from Dajjal."
4 – Disbelief and Polytheism
After the death of Hisham Ibn Abdul Malik - the Umayyad Caliph, Walid Ibn Yazid took over the reins of the Caliphate in the year 125 ah. He was of those, about whom the Noble Prophet (s.a.w) had prophesized:
هُوَ اَشَدُّ هَذِهِ الاُمَّةِ مِن فِرعَون لِقَومِه.
"From this Ummah there shall ascend to the caliphate a person, who shall be worse than what Firawn had been with respect to his people."
Walid, who was perpetually in a state of intoxication, used to say: "Who has ever said that Prophethood has been for the Hashemites? Essentially, there has neither been any Revelation nor any Book from Allah."
Once, the muezzin's recitation of the Adhan for the morning prayers aroused the inebriated Walid, who had been sleeping with his slave-girl, who too was in a state of intoxication. Waking up, he had sexual intercourse with her and then swore that he would make her lead the congregational prayers that morning. Attiring her in his clothes he sent her to the mosque in that state of ceremonial uncleanness where she led the congregation and the people offered their prayers behind her!
One day, seeking an augury (Istikhara) from the Noble Qur`an, the following verse came up before Walid: "But they sought victory and decision (there and then), and frustration was the lot of every powerful obstinate transgressor." Closing the Qur`an he suspended it as a target and then began shooting arrows at it - striking it with so many arrows that it eventually tore down into pieces. Having done this he shouted: "O' Qur`an! Do you threaten me and refer to me as an "obstinate transgressor"? When the Day of Judgment comes to pass, tell Allah that Walid tore me into pieces."
The consequence of his disbelief and rebelliousness was that he could only rule for one year - killed in an extremely horrendous fashion, his head was suspended from atop the palace and his impure body buried outside the city.
5 – Debate with the Polytheists
Prophet Ibrahim (a.s), in explaining and propagating the concept of monotheism, found himself in persistent conflict with the idolaters - who possessed idols and statues, and the star-worshippers - who claimed Allah-ship for the sun, moon and the stars, and ascribed them as partners to Allah. In Babylon and Carrhae, Ibrahim's second place of migration, they had even constructed temples and figures in the names of stars, and used to worship them!
In connection with his debate with the star-worshippers it has been narrated that once, when the darkness of the night had spread itself over the horizon and Venus had manifested itself, he (a.s) said: "This is my Lord!" When it had set he (a.s) started out in search of it but when he failed to find it, he said to the star-worshippers: "I do not love the Allahs that set."
Then he saw the moon rise whereupon he (a.s) said to the people: "This is my Lord," but when it disappeared from sight, he (a.s) said: "If my Lord does not guide me I shall surely be of the deviated ones."
When the sun rose he (a.s) said: "This is my Lord," but when it set (too) he distanced himself from the beliefs of the disbelievers and polytheists and declared: "I turn my heart and direct my worship towards He, Who has created the heavens and the earth, and I am not of the polytheists. Do you dispute me with respect to the One Allah, who has guided me aright? I fear not those, whom you regard as partners to Him."
 Surat Luqman (31), Verse 13
 Shaytan, vol. 1, pg. 697
 Ihya al-Quloob Dar Darman-e-Sifat-e-Radhilah (a work of this author), pg. 22
 Shagirdan-e-Maktab-e-Aimmah, pg. 13; Rijal Kashi, pg. 435
 Bihar al-Anwar, vol. 21, pg. 156
 Suratul Mujadilah (58), Verse 9
 Dastan-ha Wa Pand-ha, vol. 10, pg. 65, Tafsir Qur§ubi, vol. 9
 Surat Ibrahim (14), Verse 15
 Tatimmah al-Muntaha, pg. 90
 Tarikh-e-Anbiya, vol. 1, pg. 134