firstly, there's nothing that can't be surveilled. because even if the protocol is absolutely secure, they can just hack the device that it runs on which is probably far less secure than a chat program itself.
all software has bugs. one of the most secure chat protocols (OTR) have had multiple implementations, and some of these had critical issues where you could hack the device by sending a malformed chat message, no joke. it is really, really hard to write perfectly secure software. there are some examples of it being done-- there's something called formal verification-- but the things that actually do this are very small applications and would not approach the convenience and functionality of a modern desktop or phone platform.
if you are communicating stuff that is risky to your future existence you should regard encrypted communications only as buying you extra time, not an ironclad protection against surveillance. it is an extra hurdle for your adversaries to get over but given enough time and resources, if you become a priority target for some reason, they will eventually figure out how to break it
that being said, your best bets here are Signal (remember to verify safety numbers out of band) for person-to-person communication or Wickr if you want group chat.
