His real name remained completely hidden because the Priests never defined it in the first place
Your first form by which you have begun was Amun - namely, he who hides his name from the gods ... Amun is one! (He) who hides himself from the gods ... whose nature is unknown...His nature is not recorded (or displayed) in sacred scriptures; he cannot be described and taught.
He is too mysterious for his power to be laid bare; he is too great even to be asked about, too immense to be perceived. One would fall dead suddenly, in fear, if one were to pronounce the god's mysterious name, unknown to everyone. Not even a god can call him by his name, the vital one, because his name is secret.
Three are all the gods, Amun, Ra and Ptah. Aside form these there is none. He who hides his name behind the word "Amun" is Ra at his head, Ptah at his body. His cities are eternal: Thebes, Heliopolis and Memphis - forever.