There are translations of original texts. Yea i admit that i have not studied to be fluent in ancient egyptian and read the hieroglyphs and papyruses myself.
Here is one example of a text that has been translated:
en.wikipedia.org
This is not some hidden and forgotten knowledge. Sure some of the knowledge have been forgotten and the common practise was to teach simpler theology to masses that had multiple gods etc, while the ruling and priest class had more in depth studies and they studied these "gods" more of an archetypes of the Self rather than thinking that they are actual deities.
This same thing of teaching the masses different stuff is not uncommon. Christians hid most of the truth from bible. Regular jews dont understand crap about their religion compared to a rabbi who studied kabbalah well. The same thing was with ancient sumerians etc. Im not sure if the church even knows the truth anymore even at the top level, i highly doubt, but they seem to teach one thing to people and then use symbolism that points to for example egyptian origins. Like the for example the pinecone on the popes staff and large statue of it at the vatican.