If you’re in need of having a completely secure OS X system, or you’re installing a new Mac in an environment that needs an impenetrable computer, then look now further than the NSA’s very own (unclassified) Apple Mac OS X v10.3.x “Panther” Security Configuration Guide. This guide is more than enough for setting up a completely secure operating system.
For more information on setting your Open Firmware password, read Full Security. For more information on sending and receiving secure emails, read Secure Your Email.
Note: There is no such thing as a “completely secure system”, except for the one you use a paperweight or door-jam. These guides, however, will get you as close as possible to having a “completely secure system”.