WHY JUST ONE GOD?
The idea of a Supreme Power who is the First Cause of all things, the Creator and Ruler of heaven and earth has always been part of human nature from the beginning. He was not represented by images and had no temple or priests in His service. He was too exalted for an inadequate human cult. Hence there had been a primitive monotheism before people had started to worship a number of deities. Generally He faded from the consciousness of his people who formed images of many deities, His assistants, thus began the paganism. The name given to this divine Supreme Creator and Sustainer in English is GOD, Allah in Arabic. The belief of a Supreme deity who created the world and governs it, still remains among the primitive African tribes.
Why it is important to rationally believe in Just One God?
For the hope to ultimately achieve salvation!
The hope for salvation is provided:
"Surely, God does not forgive that a partner is ascribed to Him, and He forgives anything short of that for whomsoever He wills. Whoever ascribes a partner to God commits a terrible sin".(Qur'an;4:48)
The belief in One God is not to thrusted upon others, its matter free choice.
The purpose of verse (4:48) is not to tell man that he may commit any sin as long as he does not associate others with God in His divinity. The object is rather to impress upon those who had begun to regard polytheism as a trivial matter that it constitutes the most serious offence in God's sight, an offence so serious that while other sins may be pardoned this will not.
"God is the Sovereign in the Universe. Whoever ascribes partners to God, or divinity to any of His creation, has indeed invented a tremendous sin. God will forgive any transgression but polytheism (SHIRK). Included in this category are those who worship their own desire (45:23), those who indulge in human worship and in sectarianism (30:31), (42:21), and those who follow man-made books in lieu of the Book of God (6:122), (6:137), and those who claim or believe in Revelation after the Qur'an, in any form, including claims of attaining Divine knowledge through mystical experience (6:62), (7:173), (7:191). And those who uphold Trinity (4:171), (5:72-73.) And those who claim that God has a son, and those who follow their religious leaders.
"They take their rabbis, priests and monks to be their Lords* besides God. And they take as their Lord, the Messiah son of Mary. Yet they were commanded to worship but One God. There is no god but He. Praise and Glory to Him, He is far too Glorious for what they ascribe to Him. (*Talmud shows the Jewish superstition of ascribing divinity to Ezra, rabbis, saints, doctors of law and learned men. And the doctrine of Papal infallibility, divinity of saints and praying to them are common in the Roman Catholic Church).(Quran;9:31)
Such people fall from the high station of humanity. Worshiping any entity other than God, sinks the human "Self" down to subhuman levels (22:31). But most of those who claim belief (and call themselves Muslims), indulge in SHIRK (12:106).