“Open your eyes,” the online post began, claiming, “Many in our govt worship Satan.”

That warning, published on a freewheeling online message board in October 2017, was the beginning of the movement now known as QAnon. Paul Furber was its first apostle.

The outlandish claim made perfect sense to Mr. Furber, a South African software developer and tech journalist long fascinated with American politics and conspiracy theories, he said in an interview. He still clung to “Pizzagate,” the debunked online lie that liberal Satanists were trafficking children from a Washington restaurant. He was also among the few who understood an obscure reference in the message to “Operation Mockingbird,” an alleged C.I.A. scheme to manipulate the news media.