Philosophers, theologians, believers and nonbelievers from a broad spectrum of cultures and faiths have been arguing about God's existence for centuries. In Freud’s Last Session, playwright Mark St. Germain crystallizes the essence of the debate, creating a fictional encounter between Sigmund Freud, the founder of psychoanalysis and a famous skeptic, and Irish-born C.S. Lewis, a scholar, novelist and devout Christian (whose messages of faith, broadcast via radio to his fellow Britons throughout World War II, served as comfort for many during those bleak times).

The play, directed by Robert Mandel and inspired by a book by Dr. Armand Nicholi, The Question of God, is set on Sept. 3, 1939 — the day that Hitler invaded Poland, and two weeks prior to the death of Freud, who, suffering inoperable cancer, ended his own life assisted by his doctor and his daughter Anna.

The faceoff between the 83-year-old Freud and Lewis takes place after the doctor (Martin Rayner, who originated the role in 2010) invites Lewis (Martyn Stanbridge) to visit him at his home in London. A vigorous, outgoing man in his middle years, Lewis arrives under the impression that Freud has read one of his books and taken umbrage at his description of a character very like himself — “a vain, ignorant old man.” But it turns out that the ailing intellectual icon couldn’t care less what Lewis thinks of him; he’s only interested in uncovering what prompted his guest, once a professed nonbeliever, to do a 180-degree turnaround to become a man of faith. He pointedly inquires “why a man of your intellect … abandoned truth and embraced an insidious lie.”

A dialectic ensues, which examines whether morality and a conscience are inbred or taught — or whether they exist at all; also, how the notion of God is often twisted for evil ends (Hitler’s claim that crushing the Jews is “the will of God”); and — this is a thorny one — why an almighty deity allows horrific events to occur. Freud’s concept of God as the projection of infantile need and his theory that the Jews cannibalized Moses and have been expiating that crime ever since come up against Lewis’ ingenuous assumption that his Creator is the source of all good, and “you don’t have to be an imbecile to believe in him.”

Eventually, the intellectual nature of their exchange segues to the personal, as the extent of Freud’s illness and pain are revealed, along with his plans (shocking to Lewis) for a speedy, self-engendered demise. The discussion becomes moot, however, when an air raid siren sounds, and both men rapidly rummage for their gas masks and together race for shelter.

From the beginning the theatrical dynamic derives less from the ideas that are bandied about than from the characters and their contrasts. And the actors serve it well — Rayner, intimidatingly authentic as a smug, brittle, brilliant Freud, who minces no words in his takedown of others, less from cruelty than his own implacable vision of reality; and Stanbridge as the more open-minded and charitable Lewis, who overlooks Freud’s affronts and springs to action at the moment of crisis.

Of course no one wins this debate. But Pete Hickok’s period-piece replica of Freud’s study, with its imposing books and collection of artifacts, frames it handsomely, while Derrick McDaniel’s artful lighting adds a final poignant touch.

GO! Odyssey Theatre, 2055 S. Sepulveda Blvd, West L.A.; (310) 477-2055, OdysseyTheatre.com; through March 4.

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