I might be wrong, but as I understand it, you don’t need to trust a relay. It’s just a messenger that brings you notes, and the notes are signed by the private key of the account who wrote it. 

I imagine it’s like PGP signed emails. The signature is what guarantees you that you’re reading the original and unaltered email and it does not matter how you received it. 

Please correct me if I’m wrong, #nostr experts.