The first Roman Catholic appointed UK ambassador to the Vatican since the Reformation has presented his credentials to Pope Benedict XVI.
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, County Down, has worked in Rome as first secretary.
He is the first Northern Irish Catholic to hold a UK ambassadorial post since the Republic of 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 advisor on European affairs to Prime Minister Tony Blair, David Willey said.
Mr Campbell said in a speech to the Pope: "The United Kingdom is a strong supporter of furthering dialogue among peoples, in particular furthering engagement within the Christian family and with other faiths."
The job was advertised in a newspaper in July - the first time an ambassadorial post had been put out to open competition in an advert.
In a speech welcoming Campbell to the Holy See, Pope Benedict also stressed the importance of dialogue between the Christian denominations.
He said: "I encourage all those involved in this work never to rest content with partial solutions but to keep firmly in view the goal of full visible unity among Christians which accords with the Lord's will for his Church."
The previous ambassador, Kathryn Colvin, left in September after three years at the Vatican.