Term for Only "Unbelieved Warner"

Because I know my Greek mythology, metaphoric Cassandra is perfectly accessible to me. But it's a bit "literary", and I wouldn't normally use it in conversation because many people wouldn't know what I meant. In my opinion you're much more likely to be understood if you say someone is...

a [lone] voice [crying] in the wilderness
An individual advocating a course of action or proclaiming a message, who is unheeded; an individual whose opinion is proved to be right despite being ignored or contradicted by others

It's from John the Baptist in the Bible saying "I am the voice of one crying in the wilderness" when proclaiming the imminent coming of Christ, but even people who don't know that will normally be familiar with the more general use.

By way of illustrating the difference in prevalence beyond the dreaming spires of academia, check out Google results for "farage" "voice in the wilderness" and "farage" "cassandra" (Nigel Farage is a politician who's long advocated Britain's withdrawal from the European Union, but heretofore has been largely dismissed/ignored by the political establishment). Obviously there's a "gender clash" in calling Farage a Cassandra, but I'd say the main reason newspapers etc., aren't likely to do this is simply because they know many of their readers won't get the reference.

Such a person is often termed a Cassandra.