"In an unrelated web search this afternoon I came upon a post of yours on a secularist blog in which you stated that you hadn't been able to trace the origins of the trilemma, and that you had asked Purtill, Kreeft, and Beversluis for its origin but they did not know. Your conjecture that it was an anti-Arian argument is, I think correct; in this form I have traced it back to the mid 300s. See Marius Victorinus Afer, De Gener. I, p. 1020 c (Migne):
Haec dicens Deus fuit, si mentitus non est: si autem mentitus est, non opus Dei omnimodis perfectum
Roughly, 'If it is not a lie, then he was God as he said: but if on the other hand it is a lie, the works of God are not perfect (or, complete) in every way.'
Thanks Tim. I had thought I would find a trilemma in Athanasius' De Incarnatione, since Lewis wrote the preface to a translation of it, and was a little disappointed to discover it wasn't there. You have to remember that Lewis never referred to it as the Trilemma, he called it aut deus aut homo malus, Latin for Either God or a Bad Man. The first I ever saw the word "trilemma" used was by Josh McDowell.