Taken with permission from the English translation of Hujjat Allah al-Balighah by Prof. Marcia K. Hermansen.

The most basic principle of piety and the mainstay of its various aspects is the doctrine of the Unity of God (tawhid). This is because humility before Lord of the worlds, which is the greatest of the virtues in acquiring ultimate felicity, depends on this. This is the basis of intellectual regulation of [acquiring virtue] which is the more useful of the two regulations. Through this man is able to help himself address completely to learning from the Unseen, and prepare his soul to become connected to it1 through becoming sanctified. The Prophet (Allah bless him and give him peace) has informed us of its importance and its being one of the types of piety that is comparable to the human heart, which when it is sound, everything is sound, and when it is corrupted, all is corrupt. He also indicated its importance when he used the phrase about a man who died without associating anything else with God, “he entered Paradise,” or that “God prevented him from going to Hell,”2 or that “he was not veiled from Paradise,” and other expressions of this sort. He reported from his Blessed and Exalted Lord, “Whoever meets Me [on the day of Judgment] with enough sins to fill the earth, but not associating another thing with God, I shall meet him with one [an earth] like it full of forgiveness.”3

You should know that there are four degrees (maratib) of tawhid:

The first is the restriction of necessary existence (wujub al-wujud) to Him, may He be exalted, for none other than He is Necessary.
The second is the restriction of the creation (khalq) of the Throne, the Heavens, the earth, and the rest of the substances to Him, may he be Exalted. These two levels were not discussed in the divine scriptures, and the Arab polytheists, Jews, and Christians did not disagree about them; rather Magnificent Qur’an asserted that these were premises accepted by them.4
The third is restricting the management (tadbir) of the heavens and earth and what lies between them to Him, may He be Exalted.
The fourth is that no one besides Him is entitled to be worshiped (‘ibadah), and these two are intertwined and inseparable due to a natural connection between them.
Hujjat Allah al-Balighah (The Conclusive argument from God)

Attached to the Unseen, through connection to the angels. [↩]
It is narrated on the authority of Mu’adh b. Jabal (may Allah be pleased with him) that the Messenger of Allah (Allah bless him and give him peace) said: Mu’adh, do you know the right of Allah over His bondsmen? He (Mu’adh) said: Allah and His Apostle know best. He (the Messenger of Allah) said: That Allah alone should be worshiped and nothing should be associated with Him. He (the Holy Prophet) said: What right have they (bondsmen) upon Him in case they do it? He (Mu’adh) said: Allah and His Apostle know best. He (the Holy Prophet) said: That He would not punish them. (Sahih al-Muslim)
Narrated Anas (may Allah be pleased with him): I was informed that the Prophet (Allah bless him and give him peace) had said to Mu’adh, “Whosoever will meet Allah without associating anything in worship with Him will go to Paradise.” Mu’adh asked the Prophet (Allah bless him and give him peace), “Should I not inform the people of this good news?” The Prophet replied, “No, I am afraid, lest they should depend upon it (absolutely).” (Sahih al-Bukhari) [↩]
Mishkat al-Masabih, p.494 [↩]
For example the Qur’anic verse, “If you were to ask them who had created the heavens and the earth, they would say the Exalted, the All-Knowing, created them.” [↩]