In a Hubbard Communications Office bulletin, L. Ron Hubbard claims:
A THETAN is, of course, a Scientology word using the Greek “Theta” which was the Greek symbol for thought or life.
Source: HCO BULLETIN OF 5 FEBRUARY 1970, HUBBARD COMMUNICATIONS OFFICE, L. RON HUBBARD
I find this very hard to believe. The only symbolic use of the letter I know of is as an abbreviation of the word "θάνατος" (thanatos, death) when voting for the death penalty in classical Athens.
I do not consider Mr. Hubbard's writings a source of historically accurate information. However, it seems that a significant amount of people believe he knew what he was talking about. Hence, the question: Was the letter theta used to symbolize "thought or life", or anything other than death at any point in ancient Greece?