The Prophet (saas) Warned Against Idolatry

One of the most important errors that the Prophet (saas) has warned against is associating partners with Allah (shirk). As can be seen from the verses of the Qur'an, the Prophet (saas) always told people that Allah is the only god, that nothing had any power apart from Allah, and warned them against idolatry. He was commanded to do this in many verses, some of which read:
Say: "This is my way. I call to Allah with inner sight, I and all who follow me. Glory be to Allah! I am not one of the idolators!" (Surah Yusuf: 108)

Say: "I call only upon my Lord and do not associate anyone else with Him." Say: "I possess no power to do you harm or to guide you right." Say: "No one can protect me from Allah [if I should disobey] and I will never find any refuge apart from Him." (Surat al-Jinn: 20-22)
Say: "I am commanded to worship Allah, making my religion sincerely His. And I am commanded to be the first of the Muslims." Say: "I fear, were I to disobey my Lord, the punishment of a terrible day." Say: "It is Allah I worship, making my religion sincerely His, so worship anything you will apart from Him!" Say: "The real losers are those who lose themselves and their families on the Day of Rising." Is not that clear loss? (Surat az-Zumar: 11-15)
The Prophet (saas), who reminded his people that Allah was the only god, also told them that nothing they might set up alongside Allah could ever create anything nor had it any power to harm or benefit any. The verses regarding this are as follows:
Say: "Call on those you make claims for apart from Him. They possess no power to remove any harm from you or to change anything." (Surat al-Isra': 56)

Say: "Have you thought about those you call upon apart from Allah? Show me what they have created on the earth. Or do they have a partnership in the heavens? Produce a Book for me before this one or a shred of knowledge if you are telling the truth." (Surat al-Ahqaf: 4)

Say: "Call on those you make claims for besides Allah. They have no power over even the smallest particle, either in the heavens or in the earth. They have no share in them. He has no need of their support." (Surah Saba': 22)

Most people believe in Allah but are unable to appreciate His power and greatness, fail to comprehend that He is their only friend and helper, and wrongly believe that things other than Allah can be of assistance to them. The Prophet (saas) explained these truths to his people and attempted to free them from idolatry. He is commanded in the verses:
If you ask them, "Who created the heavens and the earth?" They will say, "Allah." Say: "So what do you think? If Allah desires harm for me, can those you call upon besides Allah remove His harm? Or if He desires mercy for me, can they withhold His mercy?" Say: "Allah is enough for me. All those who truly trust put their trust in Him." (Surat az-Zumar: 38)

Say: "Who is going to shield you from Allah if He desires evil for you or desires mercy for you?" They will find no one to protect or help them besides Allah. (Surat al-Ahzab: 17)
Allah created man, and is his only friend and helper. Yet, people who have no fear of Him refuse to accept that. Only in times of difficulty, when something happens to clearly show them how helpless they are, do they understand that only Allah can help them. The Prophet (saas) also reminded people of that same fact. He said that in times of trouble or loss people have no other helper than Allah. The Prophet (saas) is called upon to remind people of that in the Qur'an:

Say: "What do you think? If Allah's punishment were to come upon you or the Hour, would you call on other than Allah if you are being truthful?" It is Him you call on and, if He wills, He will deliver you from whatever it was that made you call on Him; and you will forget what you associated with Him. (Surat al-An'am: 40-41)

Believers who remember that truth emphasized by the Prophet (saas) know, without having to experience such difficulties, that only Allah can ever help them, and that only He has the power to lift their troubles and burdens. In the Qur'an, Allah says:

Say: "Who rescues you from the darkness of the land and sea? You call on Him humbly and secretly: 'If you rescue us from this, we will truly be among the thankful.'" Say: "Allah rescues you from it, and from every plight. Then you associate others with Him." (Surat al-An'am: 63-64)

The Prophet (saas) has warned all Muslims against idolatry in the hadiths:
"The most severe sin is to associate partners with Allah (known as shirk), while He has created you." (Sahih Bukhari and Muslim)

He also stressed what a great hidden danger idolatry is:
"Should I not inform you of that which I fear for you even more than the dangers of Dajjal? It is the hidden idolatry: A person stands to pray and he beautifies his prayer because he sees the people looking at him." (Sunan ibn Maajah, Volume2, Number 3389)
"Idolatry is more hidden in my Ummah than the creeping of ants across a great smooth stone on a black night..."(Tirmidhi Hadith)
Elsewhere, the Prophet (saas) stated that not associating others with Allah is both the proof as well as the necessary condition of faith:
"Whoever meets Allah, without associating partners with Him, will enter Paradise." (Kitaab At-Tawheed, Chapter: 2)