I am looking for an answer within Catholic doctrine.
Socrates, whom many consider to have undergone Baptism by Desire, had a dialogue about polytheism which I shall paraphrase below:
Socrates: Do you believe that there is an objective set of morals?
S: How would you define morals?
P: That which pleases the gods.
S: Do the gods not disagree?
P: Of course they do. [Trojan War, most of Greek mythology, etc.]
S: If they can disagree, then which god does morality please?
Socrates effectively proves that monotheism is the only theism with self-consistent non-relative morality. Atheists can act morally following Kant's Categorical Imperative, but a theist must naturally accept that morality must be defined by the creator(s), and therefore polytheism in which deities disagree cannot be self-consistent.
Why does this not apply to Catholicism? It was a heresy in the early church to say that the Three Persons in One God had only one will, as all people have individual wills, and God has three distinct persons. These wills can disagree, as is the case when Jesus prays to the Father, asking to not be crucified.
I am a high-school student, so please don't assume too much knowledge about biblical philosophy in your answer.