A paper-only journal would defend against the state, but not against people you live with. A digital journal can be encrypted, but an intelligence agency could potentially gain access (like, them reading your anti-government rants that may involve violence… that sort of stuff).
So… how to defend against both threats?
(Also, I just realized, paper journals cannot really be easily backed up…)
There are no shortage of encryption software in the market, use one, any one, Veracrypt is fine, so is million other software for this.
Use a note taking app like Joplin, Obsidian, though not open source is just as fine. Sync its data folder to the encrypted vault created with encryption software of your choice, and you are good to go
Finally my moment to shine with incredibly niche knowledge!
Joplin, while it has the ability to encrypt the sync target (even if it’s a local folder synced with syncthing) does decrypt the content in the app data folder. The notes are in an unencrypted database while all attachments just hang out in the attachment folder.
This leaves the content vulnerable if the computer is compromised. But then again, apps that keep stuff encrypted at rest still have to decrypt it to memory - leaving the content vulnerable if the computer is compromised. 🤷♂️
All in all, Joplin is definitively one of the great, more secure note taking apps.