The CS Lewis Jubilee Festival is taking place over four days to coincide with the 50th anniversary of the writer's death.
The festival has been organised by Holy Trinity Church, Headington Quarry, where Lewis worshipped for some 30 years and is buried.
Opening the festival was a talk on the life of Lewis by Professor Alister McGrath, whose biography, CS Lewis: A Life, was published earlier this year to critical acclaim.
Holy Trinity Church was packed for the talk, which charted Lewis's life and spiritual journey from aggressive atheist to Christian convert and apologist.
"Lewis is now read by more people today than during his lifetime. What makes people keep reading him?" said McGrath.
Answering his own question, McGrath ranged over the 'three Lewises' - Lewis the Oxford don, Lewis the Christian writer, and Lewis the creator of Narnia.
"The latter two are why he is remembered," said McGrath, a professor of theology at King's College London.
In addition to The Chronicles of Narnia, Lewis's best known writings include The Problem of Pain, A Grief Observed and Mere Christianity.
McGrath praised Lewis for his skill in explaining the Christian faith in a way that "made sense" while still managing to "engage the imagination".
"Lewis does need to be heard," he said.
On Narnia, McGrath said academics were still unsure as to what motivated Lewis to write a series of children's books seeing as he did not have children of his own and there was, he asserted, some evidence to suggest he did not particularly like children.
"Maybe Lewis is saying: I wish I had this kind of thing when I was younger, I might not have lost my faith," he speculated.
Whatever the reason, Narnia continues to be read widely by people young and old around the world today, and was recently introduced to a whole new generation through the big screen movie adaptations.
"The Chronicles of Narnia has stood the test of time remarkably well," he said.
The talk was followed by a time of discussion during which McGrath noted a resurgence of interest in Lewis.
This was partly due to the enduring popularity of Lewis's writings, he said, but also because of his fascinating personal life, which included the death of his mother as a young boy, the experience of trench warfare during the First World War, and the difficulty of caring for his brother, Warren, an alcoholic, and his companion, Mrs Jane Moore, as she succumbed to dementia towards the end of her life.
"People are beginning to realise [Lewis's life] is very interesting," said McGrath.
"He was a human being who actually had a rough deal in life but still managed to do some great things.
"God was able to use Lewis in wonderful ways and that is an encouraging thought."
The festival continued on Friday with a tour around Lewis's Headington that took in the former home of his wife, Joy Davidman, and the Lewis home, The Kilns.
The programme of events includes performances of a new play by Susie Stead, "Through the Wardrobe Door: The Life of CS Lewis", a Narnia-themed children's activity day, guided tours of Headington and Holy Trinity Church, and a celebration service on Sunday with guest preacher the Bishop of Oxford, the Right Reverend John Pritchard.
The festival has been co-sponsored by The Masons Arms, the local pub frequented by the Lewis brothers, which has created an ale specially for the festival called Jack's Delight, a reference to the name Lewis preferred over his real name, Clive.
One member of the audience, noting Lewis's aversion to fame and "fuss", asked McGrath what he thought Lewis would make of a festival in his honour.
He quipped: "It's right to celebrate Lewis because there is so much to celebrate - whether Lewis likes it or not!"