Do what thou wilt shall be the whole of the Law.
The word "Thelema" is commonly pronounced in two ways. This is because it is originally a Greek word, and the second letter "e" is the Greek letter Eta, which can be pronounced differently depending on whether one is emulating Modern or Classical Greek. (The first vowel is Epsilon, pronounced as a slightly longer "e", as in the English word "play".)
They-LEE-mah : corresponds to Modern Greek, pronouncing the Eta as a long "ee", as in the English word "free" (/i/ in international phonetic notation).
They-LEH-mah : corresponds to Classical Greek, pronouncing the Eta as a short "e", as in the English word "bed" (/ɛ/ in international phonetic notation).
Note that both of these pronunciations stress the second syllable, as is common in the English-speaking world (and the way Crowley pronounced it). In both Classical and Modern Greek, however, the stress would be on the first syllable, thus:
THEY-lee-mah (Modern Greek)
THEY-leh-mah (Classical Greek)
I am not a linguistics expert, so if anyone would like to offer any refinements to this page, or better yet, audio files of the different pronunciations, please contact me.
Love is the law, love under will.