When it comes to digital security, the only way to be sure that online services and platforms do what they claim to do is by auditing the code itself. This is why, generally, you should trust open-source software more than proprietary software: because anyone can review the code, vulnerabilities are identified and fixed much quicker than if the code was kept secret.
The security and privacy of Tella users is at the core of our mission. It is therefore natural for the application's code to be entirely open-source. This means that it is publicly available for review: anyone can audit our code to verify that we don't misuse user data, that we actually do what we say, and that our code is robust.
Tella's code can be accessed here on our GitHub repository. Rather than blindly trust us, audit the code yourself or ask someone you trust to do it. If you don't know someone who can audit our code, check out the report of the security audit conducted by Cure53, a security firm specialized in finding vulnerabilities. The report is available here on Cure53's website (note that at the audit was done on "Whistler", from which Tella was forked).