During his homily at Casa Santa Marta, Pope Francis explained that there are two attitudes needed to be merciful. The first is to acknowledge and feel shame for one's sins, and the second to have a "large heart.”
"If all people, individuals, families, neighborhoods, had this attitude, how much peace there would be in the world – how much peace in our hearts! Because mercy brings us peace. Always remember: ‘Who am I to judge? Have shame and enlarge your heart. May the Lord give us this grace.”
The Pope added that people with a generous heart don't judge others based on their sins, but rather, they acknowledge their own, and ask God for forgiveness.
EXCERPT FROM THE POPE'S HOMILY
Source: Vatican Radio
"It is true none of us have murdered anyone, but many little things, many daily sins, every day… and when someone thinks: ‘ But what a small heart: I have done this against the Lord!’ And he is ashamed! He is ashamed before God, and this shame is a grace: it is the grace of being sinners. ‘I am a sinner, and I am ashamed before You and ask your forgiveness.’ It is simple, but very difficult, to say: ‘I have sinned.’”
"Make the heart grow! ‘But I myself am a sinner.’ ‘Who am I to judge?’ This statement, ‘Who am I to judge this? Who am I to gossip about this?... Who am I, who have done the same or worse?’ The heart grows! And the Lord says, ‘Judge not, and you will not be judged! Condemn not, and you will not be condemned! Forgive, and you will be forgiven! Give, and you will receive!’ This [is] generosity of heart! And what will you receive? A good measure, pressed down and overflowing will be poured into your lap. And the image of the person of the person who goes out to collect the wheat with the apron and makes the apron larger so as to receive more, more wheat. If you have a wide, large heart, you can receive more.”
"Merciful men and women have a wide, wide heart: always forgiving others and thinking about their [own] sins. This is the way of mercy for which we must ask. But if all of is, if all people, individuals, families, neighborhoods, had this attitude, how much peace there would be in the world – how much peace in our hearts! Because mercy brings us peace. Always remember: ‘Who am I to judge? Have shame and enlarge your heart. May the Lord give us this grace.”