In cyber security there's no such thing as 100% secure. There's insecure, secure, less secure, more secure etc. Always relative because security is not a concrete thing. You can't at any point say this app is 100% secure. That doesn't mean that the app is not 100% secure or that the app shouldn't use encryption since it can't be proven.

This is why I like what the folks over at Notesnook are doing with Vericrypt [0], which essentially allows users to verify the encryption themselves in a very easy to use way.

[0] https://vericrypt.notesnook.com/