In the name of Allāh, the Most Gracious, the Most Merciful;
All the praise and Thanks is due to Allāh, the Lord of al-‘ālamīn. I testify that there is none worthy of worship except Allāh, and that Muhammad, Sallallāhu ‘alayhi wa sallam is His Messenger.
It is the belief that Allah is One, without partner in His dominion and His actions [Rububiyyah], One without similitude in His essence and attributes [Asma wa Sifat], and One without rival in His divinity and in worship [Uluhiyyah or 'Ibadah]. These three aspects form the basis for the categories into which the science of Tawhid has been traditionally divided.
The three overlap and are inseparable to such a degree that whoever omits any one aspect has failed to complete the requirements of Tawhid. The omission of any of the above mentioned aspects of Tawhid is referred to as "Syirik" [lit. sharing]; the association of partners with Allah, which, in Islamic terms, is in fact idolatry.
The three categories of Tawhid are commonly referred to by the following titles:
1. Tawhid Ar-Rabbubiyah [lit. "Maintaining the Unity of Lordship"]
2. Tawhid Al-Asma Was-Sifat [lit. "Maintaining the Unity of Allah’s Names and Attributes"]
3. Tawhid Al-'Ibadah [lit. "Maintaining the Unity of Allah’s Worship"] 
The division of Tawhid into its components was not done by the Prophet SAW or by his companions, as there was no necessity to analyze such a basic principle of faith in this fashion. However, the foundations of the components are all implied in the verses of the Qur’an and in the explanatory statements of the Prophet SAW and his companions, as will became evident to the reader when each category is dealt with in more detail later in this chapter.
The necessity for this analytical approach to the principle of Tawhid arose after Islam spread into Egypt, Byzantium, Persia and India and absorbed the cultures of these regions. It is only natural to expect that when the peoples of these lands entered the fold of Islam, they would carry with them some of the remnants of their former beliefs. When some of these new converts began to express in writings and discussions, their various philosophical concepts of God, confusion arose in which the pure and simple Unitarian belief of Islam became threatened.
There were also others who had outwardly accepted Islam but secretly worked to destroy the religion from within, due to their inability to oppose it militarily. This group began to actively propagate distorted ideas about Allah among the masses in order to tear down The First Pillar of Iman [Faith] and with it Islam itself
According to Muslim historians, the first Muslim to express the position of man's free-will and the absence of destiny [Qadar] was an Iraqi convert from Christianity by the name of Sausan. Sausan later reverted to Christianity but not before infecting his student, Ma'bad Ibn Khalid Al-Juhani from Basrah. Ma'bad spread the teachings of his master until he was caught and executed by the Umaiyad Caliph, 'Abdul-Malik Ibn Marwan [685-705], in the year 700 CE. 
The younger Sahabah [companions of the Prophet SAW] who were alive during this period, Like 'Abdullah Ibn 'Umar [d. 694 CE] and 'Abdullah Ibn Abi Awfa [d. 705CE], advised the people not to greet those who denied destiny nor make funeral prayers for those of them who died. That is, they considered them to be disbelievers. 
However, Christian philosophical arguments for free-will continued to find new supporters. Ghailan Ibn Muslim from Damascus studied under Ma'bad and championed the cause of free-will until he was brought before Caliph 'Umar Ibn 'Abdul-'Aziz [717-720CE]. He recanted his beliefs publicly, however, [and] on the caliph's death, he resumed teaching free-will. The following caliph, Hisham Ibn 'Abdul Malik [724-743CE], had him arrested, tried and executed. 
Another prominent figure in this controversy was Al-Ja'ad Ibn Dirham, who not only supported the philosophy of free-will, but also attempted to re-interpret the Qur'anic verses containing descriptions of Allah’s qualities according to neo-platonic philosophy. Al-Ja’ad was at one point a tutor for the Umayyad prince, Marwan Ibn Muhammad, who later became the fourteenth caliph [744-750CE]. During his lectures in Damascus, he openly denied some of Allah’s attributes, like seeing, hearing etc., until the Umaiyad governor expelled him.  He then fled to Kufah, where he continued to propound his ideas and gather followers until his heretical opinions became widely publicized and the Umaiyad governor, Khalid Ibn Abdullah, had him publicly executed in 736 CE. However, his main disciple, Jahim Ibn Safwan, continued to defend his master's doctrines in philosophical circles in Tirmiz and Balakh, when his heresies became widespread, he was executed by the Umayyad governor, Nasir Ibn Saiyar, in 743CE. 
The early caliphs and their governors were closer to Islamic principles and the consciousness of the masses was higher due to the presence of the Prophet's companions and their students. Hence, the demand for the elimination of open heretics received immediate response from the rulers. In contrast, the later Umaiyad caliphs were more corrupt and as such cared little about such religious issues. The masses were also less islamicly conscious and thus were more susceptible to deviant ideas.
As greater numbers of people entered Islam, and the learning of an increasing number of conquered nations was absorbed, the execution of apostates was no longer used to stem the rising tide of heresy. The task of opposing the tide of heresy fell on the shoulders of the Muslim scholars of this period who rose to meet the challenge intellectually. They systematically opposed the various alien philosophies and creeds by categorizing them and countering them with principles deduced from the Qur’an and the Sunnah. It was out of this defense that the science of Tawhid emerged with its precisely defined categories and components.
This process of specialization occurred simultaneously in all of the other areas of Islamic knowledge as it has done in the various secular sciences of today. Therefore, as the categories of Tawhid are studied separately and in more depth, it must not be forgotten that they are all a part of an organic whole which is itself the foundation of a greater whole, Islam itself.
1. J.M. Cowan, The Hans Wehr Dictionary of Modern Written Arabic, (Spoken Language Services Inc., New York, 3rd. ed., 1976), p.1055.
2. The word Tawhid does not actually occur in either the Qur’an or in the statements (Hadiths) of the Prophet (~). However, when the Prophet (~) sent Mu’adz Ibn Jabal as governor of Yemen in 9AH, he told him, "You will be going to Christians and Jews (Ahl Al-Kitab), so the first thing you should invite them to is the assertion of the oneness of Allah (Yuwahhidd Allah)." (Narrated by Ibn 'Abbas and collected By Al-Bukhari (Muhammad Muhsin Khan, Sahih Al-Bukhari, (Arabic-English), (Riyadh: Maktabah Ar-Riyad Al-Hadithah, 1981), vol.9, pp. 348-9, no.469) and Muslim (Abdul Hamid Siddiq, Sahih Muslim (English Trans.), (Lahore: Sh. Muhammad Ashraf Publishers, 1987), vol.1, pp.14-5, no.27). In this Hadith the present tense of the verb from which the verbal noun Tawhid is derived was used by the Prophet (~).
3. Ibn Abil-'Ezz Al-Hanafi, Sharh Al-'Aqidah At-Tahawiyah, p.78.
4. Ibn Hajar, Tahdhib At-Tahdhib, (Hyderabad, 1325-7) vol. 10, p.225.
5. 'Abdul-Qahir Ibn Tahir Ai-Baghdadi, Al-Farq Bain Al-Firaq, (Beirut: Daar Al-Ma'rifah), pp.19-20.
6. Muhammad Ibn 'Abdul-Karim Ash-Sharastani, Al-Milal Wan-Nihal, (Beirut: Daar Al-Ma'rifah, 2nd ed., 1975), vol.1, p.30.
7. Ahmad Ibn Hanbal, Ar-Radd 'Ala Al-Jahmiyah, (Riyadh: Daar Al-Liwa, 1st ed., 1977), pp.41-43.
8. Muhammad Ibn 'Abdul-Karim Ash-Sharastani, Al-Milal Wan-Nihal, vol.1, p.46.