Bismillah al-Rahman al-Rahim
TUnity or Oneness of God is one of the most fundamental foundation stones of Islam. God's unique unity is called Tauhid in Arabic. An immense and fascinating body of philosophical and mystical literature exists on this subject.
Tauhid, reduced to its most basic definition, is a word which points with commanding emphasis to the Qur'anic essential of the Oneness of God. It stands forth as a word which underlines and highlights the primary theme of the unity of the Divine which threads its way through the verses of the Qur'an, infusing the entire Book with its forceful accents and an insistent rhythm. It was inevitable that this concept of Unity which pervades every Sura with its essence would come to gain a special status within the overall framework of Islam. It is sufficient to merely glance through the Qur'an to obtain a powerful impression of this repeated and absolute insistence that, in effect, acts as the uniting theme of this Book.
But the conceptual significance of ‘tauhid’ within the Islamic world has implications which extend far beyond its fundamental definition as a religious/philosophical idea limited in its essence to the statement of the Qur'anic principle that God is One.
The unity of the Divine provides the touchstone, the underpinning upon which a wider view of tauhid as a general current coursing through the connecting conduits of the Islamic world is based. To limit the understanding of this central pillar of the Faith to its primary connotation (ie: God is One) is to fail to see it as it actually exists; and to inaccurately confine it to the realm of theology would result in the isolation of an idea which, in fact, permeates Muslim ideology on a universal level and is incorporated into historical, philosophical, sociological, and mystical dimensions of Islamic thought and attitudes.
Tauhid exists as a core concept, as the pivotal hub of a wheel whose outer rim marks the widely divergent circumference of the Islamic world and whose spokes form the supporting connections which link these varying manifestations of Islam to a common axis, to a common point of reference. To view it as merely a single variable, a single pillar, in the vast and complex equation that is Islam is to fail to fathom the conceptual weight of an idea which exists, not as an isolated factor relevant to only one sphere of perception (ie: theological / philosophical), but as a general principle which forms the underlying basis of the entire equation and, to a large degree, determines its very structure.
It is the fabric, the substratum from which every principle of the religion arises and which guides every aspect of the faith as it is propounded in the Qur'an.
“Because (God) is single, the universe is necessarily single in respect to its principle and source and in respect to its point of return and end... In consequence, just as God has no partner in essence, neither has He any partner in agency. Every agent and cause gains its reality, its being, its influence and agency from Him, every agent subsists by Him. All powers and all strength are by Him: ‘Whatever God intend, there is no strength except by Him - no power and no strength except by God.’ ”  (Mutahhari)
1 – Tauhid and God's Uniqueness
“Every choice of orientation, of an ideal, of a spiritual qibla (direction), constitutes worship. ‘Did you see the one who took his caprice (passion) for his God’ (Qur'an 25: 42).... Accordingly, tauhid in practice... means to make only God our object of obedience, destination, qibla, or ideal, to reject any other object of obedience, destination, qibla, or ideal - that is, to bow and rise for God, to stand for God, to serve God, to live for God, to die for God.” (Mutahhari)
“The greatest right of God against you is that you worship Him without associating anything with Him. When you do that with sincerity (ikhlas), He has made it binding upon Himself to give you sufficiency in the affair of this world and the next.” (Zain-al-Abideen(a.s.) Treatise on Rights)
“The first step in religion is knowledge (marifah) of Him. The perfection of knowledge of Him is to confirm Him (tasdiq). The perfection of confirming Him is to profess His unity (tawhid). The perfection of professing His Unity is sincerity (ikhlas) towards Him. And the perfection of sincerity towards Him is to negate attributes (nafy al-sifat) from Him, because of the testimony of every attribute that it is not that which possesses the attribute (al-mawsuf) and the testimony of every thing that possesses attributes that it is not the attribute.
So whoso describes God - glory be to Him - has given Him a comrade (i.e. the description). Whoso gives Him a comrade has declared Him to be two (tathniyah). Whoso declares Him to be two has divided Him. Whoso divides Him is ignorant of Him. (Whoso is ignorant of Him points to Him). Whoso points to Him has delimited Him. Whoso delimits Him has numbered Him. Whoso says, ‘In what is He ?’, has enclosed Him. Whoso says, ‘On what is He ?’, has excluded Him (from certain things).
He is a being (ka'in) not as the result of temporal origin (hadath), an existent (mawjud) not (having come) from nonexistence (adam). He is with everything, not through association (muqaranah); and He is other than everything, not through separation (muzayalah). He is active (fa'il), not in the sense of possessing movement and instruments. He was seeing when there was none of His creatures to be observed by Him. He was alone (mutawahhid) when there was none with whom to be intimate and at whose loss to feel lonely.
He originated creation and gave to it its beginning without employing deliberation, profiting from experience, occasioning movement (harakah, i.e. in Himself), or being disrupted by the cares of the soul (hamamah nafs). He delays things to their times, mends their discrepancies, implants (in them) their natural dispositions, and makes these (dispositions) adhere to their objects. He has knowledge of them before their beginning, encompasses their limits (hudud) and their end (intiha') and knows their relationships (qara'in) and aspects (ahna').” (Ali (a.s.) on God's transcendence - translated by William Chittick)