Allah (SwT) has said,
“And your Lord is One (and Only) Lord! There is no god but He” (Qur’an, 2:163).
He has also said,
“Say: He, Allah, is One” (Qur’an, 112:1).
Linguistically,”al-Wahid” means: the One Who does not socialize with people nor keeps them company. To believe in Tawhid is to believe that there is no partner at all with Allah (SwT) in His authority, and that being One is a quality of His which nobody else shares with Him.
Tawhid means recognition of the Unity of the One Who alone manages the affairs of His servants. None creates, nor sustains, nor grants, nor withholds, nor brings back to life, nor causes death, nor manages the affairs of the domain outwardly or inwardly, except Allah (SwT). Whatever He wills comes to be, and whatever He does not ever will.
Not even an atom moves without His knowledge; nothing takes place without His will. No leaf falls down without His knowledge. Nothing escapes His knowledge, not even as much as the weight of an atom in the heavens or the earth, nor smaller than that nor bigger: His knowledge encompasses everything. His might overwhelms everything. His will is affected regarding everything. His wisdom dominates everything.
Tawhid, then, means that whatever comes to your mind of how He may be or anything which you think is appropriate for Him..., He is contrary to and above it, Glory to Him.
Tawhid is the cornerstone of Islam; La Ilaha Illa Allah (SwT), There is no god but Allah (SwT). The subject of Tawhid is beyond anyone’s description, for if you discuss the Almighty, there are too many views about Him to discuss, and there are too many ways to discuss Him through Him [i.e. through His statements]. Reason recognizes Him, yet the tongue can never describe Him.
Tawhid’s meaning shatters any image and confuses all branches of knowledge, while Allah (SwT) remains just as He has always been and will always be. Glory to the One Who has made no means for His creatures to really know Him except by proving to them that they can never know Him. One who falls into the seas of Tawhid will day after day feel more and more thirsty. Tawhid is a prerogative, a privilege, of the Truth (the Almighty), yet His creatures are simply curious. Among people are those whose actions portray their belief in Tawhid; they look at everything that happens through Him. And there are those who, when the truth is unveiled before their eyes, feel less and less concerned about anyone besides Him; they see everyone to be as one secret within another...
Muhammad (S), the Messenger of Allah (S), has said,”Allah (SwT) is One, and He loves oneness.” This tradition indicates that He loves the heart that is solely dedicated to Him, Glory and Exaltation are His.
Al-Wahid, the One and Only God, protects you, the individual that you are, against the group, a number of individuals, whereas the latter cannot protect you against Him. Al-Wahid cannot be counted. He Alone is the source of all knowledge, the Only One Who reveals what is hidden. His existence has neither duration nor expiration, a limit, a dead-end, nor can anyone carry out a decision against Him, nor can His Essence ever suffer any decrease or increase whatever.