XMPP:

* supports three encryption methods (OMEMO, OpenPGP, OTR);

* has a great amount of clients [1];

* and also of servers [2];

* it is federated;

* the protocol [3] and the most established clients and servers are open;

* there are already several online services available to use [4].

And as a bonus, the famous "XMPP's myths & legends" [5].

[1]: https://xmpp.org/software/clients.html

[2]: https://xmpp.org/software/servers.html

[3]: https://xmpp.org/extensions/

[4]: https://list.jabber.at/

[5]: https://xmpp.org/about/myths.html