Saint Thomas Aquinas (1225-1274) defined the virtue of "mercy" in his great Summa Theologiae as "the compassion in our hearts for another person's misery, a compassion which drives us to do what we can to help him."
If the gospels show us God's mercy expressed in decisive acts for our salvation (such as the Incarnation, Cross, and Resurrection of His Son), the Apostolic letters in the New Testament are the praise and proclamation of that mercy, and an exhortation to practice it.
If the Son of God Himself is overflowing with merciful love, it is no wonder that the New Testament encourages everyone to place all their trust in Him, and in His heavenly Father.
"At one time, the Saint, with satisfaction, told Jesus that she had offered him all of her life and all that she had," Pope Francis said. "But Jesus’ answer stunned her."