An ordained Anglican priest, he was a proponent of the Christ myth theory, the idea that Jesus did not exist but is a fictional or mythological figure.

He went on to study literae humaniores ("greats" or classics) at Oriel College at the University of Oxford as a Rhodes Scholar from 1951 to 1954 where he read the ancient historians (Herodotus, Thucydides, and Tacitus) completely in their original texts.

Harpur also wrote a number of books on religion and theology, ten of which became Canadian bestsellers and two of which were made into TV series for VisionTV.

Harpur's 2004 book The Pagan Christ was named the Canadian non-fiction bestseller of the year by the Toronto Star and The Globe and Mail.

Harpur was a Fellow of the American Religious Public Relations Council, and in 1976 won a State of Israel Silver Medal for Outstanding Journalism.

Harpur builds on Alvin Boyd Kuhn when listing similarities among the stories of Jesus, Horus, Mithras, Buddha and others.

According to Harpur, in the second or third centuries, the early church created the fictional impression of a literal and historic Jesus and then used forgery and violence to cover up the evidence.

Having come to see the scriptures as symbolic allegory of a cosmic truth rather than as inconsistent history, Harpur concludes he has a greater internal connection with the spirit of Christ.