Archbishop of Canterbury to meet Pope Francis in defining moment for Christianity

It was measure of how warm a relationship the Archbishop of Canterbury has with Pope Francis.

When asked by a child this summer who would win if they had a fight, Archbishop Welby joked the Pope would, for sure.

"He has a bigger stick than me, and has a bigger hat than me - and he's better than me."

But whatever the size of his hat and his stick, there is no doubt that this week the head of the Anglican Communion will be welcomed with open arms by Pope Francis, when the Archbishop of Canterbury makes a historic trip to Rome.

It marks fifty years since a remarkable turning point in the relationship between the two churches.

For four hundred years since Henry VIII broke with Rome, there was no formal contact between the Anglicans and Roman Catholic faiths, a history littered with cruelty and violence, morphing slowly into mere animosity and suspicion.

Then in 1966, the year England lifted the World Cup - another reason for many Christians to rejoice. In a bid to find a path to unity after centuries of division: Pope Paul VI formally invited Archbishop Michael Ramsay to Rome.

Looking at the grainy black and white images of the event, they feel as though they are from another world - certainly one with far more regard for religion than the one we live in now.

But they chart a moment of history.

In a symbolic exchange of gifts, Pope Paul VI presented Archbishop Ramsey with a ring - a ring which Archbishop Welby, will wear when he lands in Rome late tonight.

It's a ring which all Archbishops of Canterbury have worn when in Rome since it sealed a joint effort for the two Churches to find their way back together.

In a secular world though, where few might understand or care about the finer points of doctrinal difference between Anglicans and Roman Catholics, why should it make any difference if the two Churches are trying to find ways of reuniting?

And indeed with very obvious divisions on issues of women priests and homosexuality - is there actually any genuine hope of it working?

It is no small challenge.

But both Pope Francis and Archbishop Welby (who struggles to hold his own communion of Anglican churches together) are up for it.

They are two pragmatists who will acknowledge there are nearly insurmountable differences in some areas - but who seem determined to ensure there are many ways in which the two Churches can act as one: against poverty, war, indifference and injustice.

To that end tomorrow, they will commission 19 pairs of Bishops, from the Roman Catholic Church and the Anglican Communion to go out into the world and get their hands dirty - together.

It's being heralded as a defining moment in Anglican-Roman Catholic relations.

From tomorrow I'll be reporting on the visit exclusively for ITV News as part of Archbishop Welby's delegation to Rome - to see close up how the two men work together, and what mark they can hope to stamp on a world grappling with some of its biggest challenges ever.