The first Roman Catholic to be appointed UK ambassador to the Vatican since the Reformation presented his credentials to Pope Benedict XVI yesterday.
Francis Campbell (35) stressed the UK's commitment to dialogue within the Christian family and with other religions at the ceremony.
He also paid tribute to the Catholic Church's role in providing education and healthcare in the developing world.
Mr Campbell, from Newry, Co Down, has worked in Rome as first secretary.
He is the first Northern Irish Catholic to hold a UK ambassadorial post since Ireland gained independence in 1921. BBC correspondent David Willey said heads of the diplomatic mission in Rome were normally at the end of their careers, and that Mr Campbell was "a career diplomat of a new breed".
He "has the ear of Downing Street" as a former adviser on European affairs to prime minister Tony Blair, Mr Willey said.
The job was advertised in a newspaper in July - the first time an ambassadorial post had been put out to open competition in an advertisement.
The previous ambassador, Kathryn Colvin, left in September after three years at the Vatican.