مَنْ وَحَّدَ اللّهَ سُبْحانَهُ لَمْ يُشَبِّهْهُ بِالخَلْقِ
Whoever believes in Allah’s oneness, does not liken him to his creatures.
Tawheed (monotheism) is the first and most important belief in Islam. Surat Al-Ikhlas (Surat Al-Tawheed) describes Allah:
Ali bin Abi Talib has many sermons explaining Tawheed, among them is sermon 64 in Nahjul Balagha:
Praise be to Allah for Whom one condition does not proceed another so that He may be the First before being the Last or He may be Manifest before being Hidden. Everyone called one (alone) save Him is by virtue of being small (in number); and everyone enjoying honour other than Him is humble. Every powerful person other than Him is weak. Every master (owner) other than Him is slave (owned).
Every knower other than Him is seeker of knowledge. Every controller other than Him is sometimes imbued with control and sometimes with disability. Every listener other than Him is deaf to light voices while loud voices make him deaf and distant voices also get away from him. Every onlooker other than Him is blind to hidden colours and delicate bodies. Every manifest thing other than Him is hidden, but every hidden thing other than Him is incapable of becoming manifest.
He did not create what He created to fortify His authority nor for fear of the consequences of time, nor to seek help against the attack of an equal or a boastful partner or a hateful opponent. On the other hand all the creatures are reared by him and are His humbled slaves. He is not conditioned in anything so that it be said that He exists therein, nor is He separated from anything so as to be said that He is away from it. The creation of what He initiated or the administration of what He controls did not fatigue Him. No disability overtook Him against what He created. No misgiving ever occurred to Him in what He ordained and resolved. But His verdict is certain, His knowledge is definite, His governance is overwhelming. He is wished for at time of distress and He is feared even in bounty.
لو كان لربك شريك لأتتك رسله
If there was another God he would have sent you his prophets.