Allah says what means, “If there had been in the heavens and on the earth, other deities besides Allah, both the heavens and earth would be ruined…” (Qur’an 21:22).
But what is the rationale beyond the impossibility of having more than one creator in concordance with each other and without clashing decisions?
The question is not about issuing two clashing decisions so the universe would be ruined, or about being in agreement so the universe would remain intact. The rationale presented by this previous verse is far and beyond any of this.
Having another creator besides Allah necessitates composition (multiplicity of essences and causes); and composition necessitates need. So, if we assume (Allah forbid) that the Deity has a need this would provide an insecure universe that is susceptible to collapse without any guarantees or reasons since the Deity is in need. “…So exalted is Allah, Lord of the Throne, above what they describe.” (Qur’an 21:22).
Allah is exalted above any composition or need since He is self-sufficient and self-subsisting.
Add to all this: If there were two deities (or more) the probability of disagreeing stands more to reason than concordance all the way. Having more than one essence, more than one will and more than one determination means that each one of them is in need and need necessitates convergence.
[Renounce your Atheism by Dr Haitham Talaat, p.14]
