The encrypted messages

What if someone decided one fine morning to start jotting down their own personal messages in an encrypted format of a language? It wouldn't have to be something super specific or very well thought out. However, it would be highly personalized and individualistic. The question that bothers me is would the encrypted messages ever be decrypted?

Obviously, there are a lot of encryption specialists and cryptography specialists in the world that do this for a living. I am firm in believing that there always is a system to every crypt. But would the system be found or realized if there was nothing to go off of.

Imagine the walls of caves that have been rediscovered after millenias. Most of them, and if, they are painted with texts and images are often decrypted. The languages there arent common knowledge to our modern times. But they are nevertheless decrypted and the messages are unravelled. Most of them have a system and clues which kicks off the discovery. But what if there was one without said clues and systems?