Who or what created the universe

[Ahriman]

You ask what is wrong with having a belief in something that the imagination creates.

On the surface, nothing. The issue is when the person is a danger to himself, a psychotic who belongs in an asylum, or someone who is a danger to others,  a delusional theist who wants to dictate the lives of others with subjectivity.

Most everyone agrees that children should outgrow childish things, and the same should apply to adults who should outgrow religious irrationality.

Yeah, that's basically true, and in an optimistic way, no less. But even then, religion itself isn't the problem. It's the way the known world religions are taught. If people could be taught to embrace a religion that expressed absolutely no discrimination of any kind, was intellectually honest, and provided for their material needs (in a real way, not just wishful thinking), I don't see why such a religion should not be allowed to exist and prosper.