Threema is a personal messenger which is End-to-End Encrypted and made by Threema GmBH, a Swiss company.
The server software is closed source but the client is open-source and its security has been audited two times independantly, holding particularly good results.
One of the most peculiar aspects of Threema is that this messenger is paid instead of relying on data collection, subscriptions or donations to survive.
A libre version without any proprietary addons is available on F-Droid.
Threema is priced as it follows (prices taken as of 2022/11/07):
You can get the app a few ways:
You can also install it via F-Droid but it will require a license bought on their shop.
Using Threema is as straightforward as using any other messenger.
The only notable difference is that you don't have to provide any phone number nor any other personal data to the app to function.
Instead, you will be assigned an ID that you can share with friends to be able to talk.
My personal recommendation is to go check out in the settings and tweak everything you can to suit your needs.
If you talk often with people you can meet AFK, it is also recommended to check and validate their keys by scanning their QR code with the app.