Q: Theives and a passcode.

There are several methods they could employ.

A Restore will erase everything on the phone.  If you had set Find-My-Iphone your apple ID and password would be needed for the Restore to make the phone usable.

As already mentioned, make sure to have set up "Find my iPhone" and also set up a passcode for the lock screen if you are not the only one having access to it.

iOS passcodes

Find My iPhone Activation Lock

Check this article about iCloud security and privacy overview

And, not to forget, don't jailbreak the phone:

Unauthorized modification of iOS has been a major source of instability, disruption of services, and other issues