Esperanto

It's great!  If you want to say the opposite of something, you just put "mal" before it.

theism= tiesmo
atheist= atiesmo

Wait.  What?  That's fucking stupid.  What is the point of making a prefix you don't even use?  It should either be "netiesmo" (not theist) or "maltiesmo" (strong atheist "god doesn't exist").

2 cents' worth from an Esperanto speaker:

"teismo" and "ateismo" (not "tiesmo" and "atiesmo") are a (long-standing) glitch - and an optional one at that.  You are absolutely allowed to use regular derivation to say "neteismo" and "malteismo" for "atheism".  In fact, you can take it one step further:  use regular derivation and the perfectly good existing Esperanto roots "Dio" (God) and "-ismo" (belief system) to form "diismo" (theism), and "nediismo" and "maldiismo" (atheism).  As an Esperanto speaker, I definitely prefer the fully regular and derived "diismo", "nediismo" and "maldiismo" over the other less-regular and/or less-derived forms.  I additionally dislike the word "teismo" because of its ambiguity (is it "belief in God", or is it "belief in tea" [te-o = tea, -ismo = belief system]?).

Esperanto speakers can vote on vocabulary with their feet.  Hopefully, these words are on their way out, even if they still appear in the Plena Vortaro de Esperanto.