Monotheism (from the Greek monos – single, theos – god) is the belief in the existence of just one god, to polytheism the belief in more than one god, to atheism the lack of belief in any god.
Monotheism is not about putting a partner alongside God but about worshiping God alone. ‘You alone we worship and you alone we ask for help,’ says the first chapter in the Quran, the chapter used in the ritual prayer and the chapter that is The Key (Al-Fatehah).
God tells us in chapter 2, verse 256 that, ‘There is no compulsion in religion; truly the right way has become clearly distinct from the wrong way. Therefore anyone who denounces the devil and believes in God he indeed has laid hold on the firmest grasp, which shall never break, and God is Hearing, Knowing.’
God informs us in the Quran that He is Omnipotent (2:20) unlimited in power, Omnipresent (2:115) present everywhere at the same time and Omniscient (2:32, 115) unlimited in knowledge and that God and His angels and those with knowledge (3:18) bear witness that there is no god but He. God is a truthful and equitable and absolute god and there is no god but Him the Almighty, Most Wise.
The most important commandment in any religion is to bear witness to God alone, but polytheism has bought in many gods and atheism has taken the gods away. Imagine more than one god, who would get the final decision? God says in chapter 21, verse 22 that if there were other gods beside God there would have been chaos and He claims absolute authority.
Monotheism is not a new religion or a new cult. Abraham was a monotheist and Mohamed was a monotheist because he followed the religion of Abraham. Islam means submission to one god.
Chapter 16, verse 120
Surely Abraham was an exemplar in his obedience to God, upright and he was not of the polytheists.
Chapter 42, verse 13
He decreed for you the same religion decreed for Noah, and what we inspired to you, and what we decreed for Abraham, Moses, and Jesus: "You shall uphold this one religion, and do not divide it." The idol worshipers will greatly resent what you invite them to do. God redeems to Himself whomever He wills; He guides to Himself only those who totally submit.
Chapter 6, verse 161
Say, “My Lord has guided me in a straight path, the perfect religion of Abraham, monotheism. He never was an idol worshipper.
God does not forgive idolatry, association of anything else with God. He forgives lesser sins for whichever sinner he chooses but God does not forgive idolatry if persisted until the day you die (4:48). Your death bed is not the time and place to decide finally that you believe in God and God alone.