I think there is a nice way of reconstructing Fichte's argument for the Critique of all Revelation in a mode that uses just modern communication channels and cryptology.
Basically, we have a communication channel between God and some believer, say Joseph. We also have the Devil, who is the adversary and has all the advantages that adversaries have in cryptological setups, i.e. unlimited access to prior communications, channels, etc. Notice that due to the spiritual speed at which both God and the Devil can work (Boolos 1974), the usual mathematical "trapdoor" ciphers that humans use are pointless, because spiritual entities can bring arbitrary amounts of brute force to bear on cracking the cipher.
God is trying to send Joseph a message. The Devil can intercept and substitute part or all of the message. Joseph needs to be able to determine whether the message is from God or the Devil. Since the structure of the message can be forged, the determination has to be by the contents.
The clearest choice is to say something already known. By repeating something that God has already said, Fichte basically argues, Joseph can be convinced of the authenticity of the contents. Even if the Devil repeats something God has already said, the Devil has technically ursurped the channel, but made no progress in misleading Joseph.
Notice that it is insufficient to prefix new contents with known contents; the Devil can trivially do that as well (e.g. "God created Heaven and Earth. Kick ten puppies a day").
This means that for any true believer, all revelation is already over. For Fichte this was no problem, because he was convinced that all the properties of God that mattered, esp. the moral doctrine, could be deduced logically as natural religion anyway. For revelatory religions like Mormonism, this is a big issue; there is no way at all to provide the day-to-day guidance of the type found in Doctrines and Covenants. None of them are above the suspicion that they could be substitutions of the Devil.