Since one of them is one of most self-publicised atheists in the Western world and the other is arguably the most famous living convert to the Roman Catholic Church, there may not be much of a meeting of minds. Yet Tony Blair has agreed to share a platform with the British-born journalist, Christopher Hitchens, to argue over whether religion is a good thing generally, or not.

Tickets to this unique event will be on sale tomorrow, but for anyone in Britain wanting to go, the bad news is that first they must book a seat on a plane or a liner. Mr Blair very rarely makes a public appearance in the land where he was prime minister. When he does, he usually attracts the sort of hostile demonstration that forced him to cancel last month's launch party for his new book, A Journey.

He and Mr Hitchens will go head-to- head in Toronto, where Mr Blair has fewer enemies, and the Iraq war is less of an issue. Iraq is also one of comparatively few issues on which he and Mr Hitchens agree. Mr Hitchens has consistently praised Mr Blair's willingness to send British troops to combat dictators, in Kosovo, Sierra Leone and Iraq.

On the question of religion, however, they are so far apart that the organisers revised the original idea of having them debate whether God exists. Instead, Mr Blair will speak in favour of the motion: "Be it resolved, religion is a force of good for the world."

When he was in office, Mr Blair was advised by Alastair Campbell that it was a bad idea for a practising politician to "do God". It is assumed that he decided years before leaving Downing Street that Rome was his true spiritual home, but he waited until he was out of Downing Street before switching.

Mr Hitchens, by contrast, is the author of a 2007 book much deplored by religious believers, called God is not Great: How Religion Poisons Everything – a polemic against religion generally, and Christianity in particular.