Pope Francis has revealed that an encounter with the Salvation Army when he was just four years old opened his eyes to ecumenism and Christian unity.

Speaking to a delegation of leaders from the Salvation Army in the Vatican, the Pope expressed his hope that "Catholics and Salvationists will continue to render a common witness of Christ and of the Gospel in a world in such great need of experiencing the mercy of God. It needs it!"

"Catholics and Salvationists, together with other Christians, recognize that the needy have a special place in God's heart, so much so that the Lord Jesus Christ made himself poor for us," Francis said.

"They meet often at the same human fringes, and my heartfelt hope is that our common faith in our Saviour Jesus Christ, the one mediator between God and men will become increasingly a solid foundation of friendship and collaboration between us."

The Pope is well known for his heart for vulnerable people. "I prefer a church which is bruised, hurting and dirty because it has been out on the streets, rather than a church which is unhealthy from being confined and from clinging to its own security," he wrote in his apostolic exhortation last year.

During his inauguration Mass in March 2013 he also highlighted the Christian calling to be close to "the poorest, the weakest, the least important...the hungry, the thirsty, the stranger, the naked, the sick, those in prison".

It is unsurprising, therefore, that he stressed the need to "go out to others, seek those who have fallen away, stand at the crossroads and welcome the outcast," in his speech on Friday.

"The Church which 'goes forth' is a community of missionary disciples who take the first step, who are involved and supportive, who bear fruit and rejoice. An evangelizing community knows that the Lord has taken the initiative, he has loved us first," Francis said.

"Such a community has an endless desire to show mercy, the fruit of its own experience of the power of the Father's infinite mercy."

The Pope added that theological differences between Salvationists and Catholics "must not impede the witness of our shared love for God and for our neighbour," and said that his first encounter with the Salvation Army as a young boy showed him that the two are working for a common aim.

"When I was four years old – it was the year 1940, none of you were born! – I was going on the street with my grandmother. At that time, the idea was that all Protestants were going to hell," he recalled.

"On the other side of the pavement two women of the Salvation Army were coming...And I remember, as if it were today, that I said to my grandmother: "What are they? Nuns, Sisters?" And my grandmother said: "No, they are Protestants, but they are good."

"And so my grandmother, because of your good witness, opened the door for me to ecumenism. The first ecumenical homily I had was in front of you. Thank you very much."

Francis ended with a prayer for the Salvation Army, and for its work "which enable the light of Christ to shine in the darkest corners" of people's lives.

"And I trust that you will also pray for me: I need it," he concluded.