Why Just One Creator?
Our natural disposition/fitrah tells us that there’s only one Creator . From the harmony of all systems in the universe we’re also bound to infer that there can only be one Creator . Since God is a metaphysical entity (beyond the physical universe), we have no verifiable or falsifiable way of ascertaining that our Creator has taken partners. The only possible way for us to conclude multiple creators was if God Himself somehow communicated this information to us . His claim, however is that He is alone. Quran comments on shirk and poses a challenge to mushriks that if they have any evidence of partners of God that they should bring it forward. Of course they couldn’t, because one simply doesn’t exist. Quran 35:40 Translation by Asad Say: “Have you ever [really] considered those beings and forces to whom you ascribe a share in God’s divinity, [and] whom you invoke beside God? Show me what it is that they have created on earth – or do [you claim that] they have a share in [govern­ing] the heavens?” Have We ever vouchsafed them a divine writ on which they could rely as evidence [in support of their views]? Nay, [the hope which] the evildoers hold out to one another [is] nothing but a delusion. Quran 46:4 Translation by Asad Say: “Have you [really] given thought to what it is that you invoke instead of God? Show me what these [beings or forces] have created anywhere on earth! Or had they, perchance, a share in [creating] the heavens? [If so,] bring me any divine writ pre­ceding this one, or any [other] vestige of knowledge – if what you claim is true!” The purpose of religion as per the Quran is to guide us to attain purity (tazkiya). An ideological impurity at such a fundamental level would pose the biggest challenge for us in that regard. Since polytheism is a completely baseless idea and it has such a far reaching impact, therefore it’s also deemed by God as the biggest sin.
But we will have to accept God before we accept religion. And through reason the above argument could be valid (unless I’m wrong). The only way to reject it is to first accept that religion. So to someone who the deen has not reached- if he reaches this conclusion why would he be wrong?