The Archbishop of Canterbury preached for the first time at Westminster Cathedral, the heart of the British Catholic Church, on Sunday.
Cardinal Vincent Nichols welcomed Justin Welby along with Catholic bishops from England and Wales for the closing of the Door of Mercy at the end of the Year of Mercy.
Welby said the year, which began in December 2015, had "caught the imagination, not only in the Catholic Church, but in all churches and far beyond," according to Independent Catholic News.
He added: "Mercy is the gift that goes on giving... that dissolves the hardness of hearts."
The Year of Mercy was announced by Pope Francis in March 2015 and is described as "extraordinary" because it does not fit within the usual 25-year cycle of Jubilee Years in the Catholic Church.
Each Jubilee Year is usually marked with the opening of the Door of Mercy in St Peter's Basilica in the Vatican. But for this Year of Mercy Pope Francis wanted a door in each diocese so everyone could celebrate the festival.
Welby said the Door of Mercy "calls us back to receive mercy by grace and to be people of mercy". He compared it to the wardrobe door in the Narnia chronicles and said mercy opens people to a different world – the Kingdom of God.
Vincent Nichols, as Archbishop of Westminster, is the head of the Catholic Church in England and Wales and opened the Door of Mercy on 13 December 2015.
Welby's first sermon at Westminster Cathedral for the closing of the door comes after a number of ecumenical initiatives to bring the two Churches together.
The door has remained open throughout the Jubilee Year of Mercy, receiving pilgrims from around the country. The year will end on Sunday 20 November when Pope Francis will close the Holy Door at St Peter's Basilica.
