But if his enthronement as Bishop of Durham, the fourth most senior role in the Church of England, took him by surprise it would prove to be just one step in a rapid rise.
The choice of the 56-year-old to lead the world’s 77 million Anglicans marks a decisive break with the past for the Church.
While his predecessors have drawn on long careers as academics or clerics, his experience is of the world of mammon as much as God.
A former oil executive he gave up a highly paid career after feeling a “call” to the priesthood in the late 1980s.
“Something in me just said ‘this is what you should be doing’,” he recently explained.
He was later able to draw on years of experience in oil exploration in troubled areas of west Africa, when his ministry led him to work in conflict resolution in the violent Niger Delta, where he narrowly avoided being shot dead.
At a time when the Church is grappling with the aftermath of the banking crisis, he combines – almost uniquely – an understanding of the working of the City with that of life in the inner city, gleaned as a parish priest and Dean of Liverpool.
He has used his seat in the Lords as a platform to challenge the “sins” of the multi-billion pound banks as much as the small-scale payday “loan sharks” he has seen at work on the North East – condemning the practice in the language of the Old Testament as “usury”.
Although educated at Eton and Cambridge and even a member of a Pall Mall club, he is seen as far from an establishment figure.
Theologically, he is unashamedly part of the evangelical strand of the Church, upholding a more traditional and conservative interpretation of the Bible than some.
But while he has, for example, publicly criticised the Coalition’s plans for gay marriage, he is not without support among liberals, some of whom believe he will prove a pragmatic and flexible Archbishop.
He is also a strong advocate of more modern styles of worship.
As Dean of Liverpool, where he almost doubled the cathedral congregation, he gave his blessing to a Halloween service called “Night of the Living Dead”, complete with a man in gothic dress leaping out of a coffin, to illustrate the message of resurrection.
To the surprise of some, he also allowed the cathedral bell-ringers to chime the Liverpool-born John Lennon’s Imagine, widely regarded as an atheist anthem.
Yet he is also an enthusiast for catholic styles of worship, has close links to Benedictines and regularly goes on spiritual retreats.
At a time when the Anglican Church worldwide is deeply divided between liberals and conservatives over issues such as homosexuality, what diplomatic skills he can muster look certain to be put to the test.
His family background is as colourful as some of the services he has attended. He recently discovered that his father Gavin was effectively a bootlegger, trading in whisky during the prohibition years in America.
Welby senior eventually came up with a better ruse to get around the ban – switching to selling Communion wine – and was later to move in the same circles as the Kennedy family.
Bishop Welby’s mother, Jane – a relation of Rab Butler, the former Conservative deputy prime minister – was Winston Churchill’s private secretary.
His parents separated when he was very young and boarding school was to be the backdrop to his childhood, including, famously, Eton. In stark contrast, he and his wife Caroline, a classics teacher, would go on to have a large and close-knit family.
They had six children, one of whom, their seven-month-old daughter Johanna, died in a car crash in France in 1983.
“It was a very dark time for my wife Caroline and myself, but in a strange way it actually brought us closer to God,” he said in an interview.
It was the start of the process which led him to the priesthood.
At the time he combined his work as group treasurer of Enterprise Oil with being an active member of Holy Trinity Brompton, the lively evangelical church in west London which invented the Alpha course – the short introduction to Christianity which has been followed by around eight million people around the world.
Known for his self-deprecating manner, he describes himself as having “drifted” into the oil industry because he could not get a job when he left university.
And asked in a recent interview for his comment after being tipped as the next Archbishop of Canterbury, he suggested he did not want the job describing himself as “one of the thicker bishops in the Church of England.”
But to those who know him he is anything but.
Among those who encountered him while he was training for the priesthood Cranmer Hall in Durham, he is described as “one of those students you just remember”.
He went on to serve as a curate in Nuneaton and then vicar of Southam, a small Midlands market town, where he saw the congregation treble.
He moved to Coventry where he served as director of the Cathedral’s internationally renowned reconciliation ministry. It was there that his knowledge of west Africa in particular would be put to good use and he developed skills of negotiation and dialogue which will be desperately needed within the fractious Anglican communion.
Later as Dean of Liverpool he oversaw strong growth in the congregation as well as being credited with strengthening the cathedral’s finances. One member of the congregation there said: “He spiritually (and fiscally) transformed the place and turned a moribund congregation around.”
The call for interview as possible Bishop of Durham last year came as complete surprise to him, he later said.
Facing the prospect of upheaval for his family, he took weeks to decide. But once there he set about reorganising the diocese’s finances and had begun a drive to revive dwindling congregations.
As the fourth most senior bishop in the Church, the position brought with it a seat in the Lords. Within months of his arrival he had been appointed as a member of the Parliamentary commission on banking standards, set up in the wake of the libor fixing scandal.
His contributions there recently earned him a profile in the Wall Street Journal, perhaps a first for a churchman.
But for all the dramatic changes in his life in the last year, perhaps the biggest has been much closer to home.
Earlier this autumn, as speculation swirled about his possible candidatures as Archbishop of Canterbury, Bishop Welby’s attention was on an even more dramatic change in his family life: becoming a grandfather.
He posted a message on Twitter speaking of how he felt a “powerful sense of the flow of time”.
It is a sense he is likely to feel acutely early next year as he ascends the throne of St Augustine as 105th in a line stretching back more than 1,400 years.