Dutch-born Rien Schuurhuis (40) will ride in Wollongong, Australia, wearing a yellow and white Vatican flag with crossed keys and a papal tiara.
"It is an incredible honour. I think it will fully sink in at the start," the Groningen native told AP.
"It's a big first step towards what the Pope believes can be achieved through sport: equality and fraternity. Everyone on the sports field, in this case on the road, is equal, regardless of origin, religion or age," he added.
Vatican athletes have so far appeared out of competition at events such as the Small States Games of Europe and the Mediterranean Games.
This means the world's smallest country will have its first proper competitor at the cycling championships thanks to Schuurhuis. The International Cycling Union (UCI) accepted the Vatican as its 200th member last year.
Schuurhuis is able to represent the Vatican thanks to the fact that his wife is the Australian ambassador, Chiara Porro, to the church-state.
He rode in Dutch colours until last year on the UCI continental circuit, one level below the elite World Tour. "He's a good cyclist. He raced at a high level," explained his coach Valerio Agnoli, a former teammate of Grand Tour winners Ivan Basso and Vincenzo Nibali.
"I knew how to ride a bike before I could walk," Schuurhuis recalled of his origins in the land of bicycles.
The amateur cyclist trains on Rome's busy roads, but not in the city-state whose colours he represents. "I tried riding there once with my son. But we were reprimanded by the police because you're not allowed to cycle in St Peter's Square," he recalled with a smile.
Schuurhuis doesn't expect to be fighting with the best on Sunday. His main goal is to spread the Pope's message. "When people see his special jersey, they get curious," said Agnoli, whose charges, for example, were sought out by star Belgian Wout van Aert during training in Australia.
With the race starting at 2:15am Rome time, it's not likely that Schuurhuis will be watching Pope Francis on television.
"The time difference is a problem. But maybe he will watch the footage," said Giampaolo Mattei, president of the Athletica Vaticana organisation.
The Pope as well as the rest of the world can follow the race with Flashscore.