the concept of revelation means it must exist

The concept of "Revelation" (communication from the Necessary Being, especially regarding moral principles) means that it must exist.

We abstract in our minds certain impressions of reality, and these impressions are rational if and only if they cohere with our intuitive experience.
As such, anomalies necessitate evidence for the impressions we hold.
The anomaly of "Revelation" disrupts the coherence of the apparent impression "no revelation";
therefore, evidence must be produced—of something that doesn't exist (which is rationally impossible)—to justify the impression of "no revelation" against the impression "revelation exists".

As such, the concept of revelation is proof of its existence and it must exist.

Suppose someone objects, in regards to substituting "unicorn", for "revelation"; although any statement is true in any possible world, this is describing something else entirely—which would be the fallacy of False Equivalency.
Unicorns are created and Revelation is not; the former is contingent, and as such, you would have to specify in what framework it is contingent (contingent beings such as unicorns made of material occupy some space).
If you wanted to specify "on earth" then it would be rather implausible that they exist, but regarding "everywhere" this basically can't be confirmed that they do not.

Furthermore, there appears to be several religions that claim "Revelation", and without ascertaining the epistemological certainty of the claims, they still give the appearance of massively successive claims of testimony. As such, we can conclude that the impression "Revelation", could not be disrupted by the coherence of the impression of "no revelation (and/or no moral guidance)", due to these massively successive claims of testimony to the concept.