Pope Francis urged Church leaders to be more welcoming in a sermon at St Peter's Basilica on Sunday. 

Speaking before hundreds of cardinals and bishops, Pope Francis warned against having a "prejudiced mentality" that marginalises others.

"There are two ways of thinking and of having faith: we can fear to lose the saved and we can want to save the lost," he explained. "Even today it can happen that we stand at the crossroads of these two ways of thinking."

The Pope used the biblical story of Jesus healing a leper as an example of how Christians should treat society's outcast. 

"Jesus responds immediately to the leper's plea, without waiting to study the situation and all its possible consequences," Francis said.

"For Jesus, what matters above all is reaching out to save those far off, healing the wounds of the sick, restoring everyone to God's family.

"And this is scandalous to some people! Jesus is not afraid of this kind of scandal," he continued. 

He does not think of the close-minded who are scandalised even by a work of healing, scandalized before any kind of openness, by any action outside of their mental and spiritual boxes, by any caress or sign of tenderness which does not fit into their usual thinking and their ritual purity."

Pope Francis has consistently called for unity against injustice, and has made the plight of underprivileged populations, exploited workers, and victims of modern slavery central causes for the Vatican. 

"Total openness to serving others is our hallmark, it alone is our title of honour!" the pontiff proclaimed. 

"We will not find the Lord unless we truly accept the marginalized! Truly the Gospel of the marginalised is where our credibility is at stake, where it is found, and where it is revealed."