Mercy is not a principle to learn, but it is an apprenticeship for the mature believer:
Jesus Christ taught that (humankind) not only receives and experiences the mercy of God, but that (we are) also called “to practice mercy” towards others: “Blessed are the merciful, for they shall obtain mercy.”(Mt. 5:7) The Church sees in these words a call to action, and she tries to practice mercy. All the beatitudes of the Sermon on the Mount indicate the way of conversion and of reform of life, but the one referring to those who are merciful is particularly eloquent in this regard. (Humankind) attains to the merciful love of God, His mercy, to the extent that (we ourselves are) interiorly transformed in the spirit of that love towards (our) neighbor.
We may not always succeed in practice, but St John Paul encourages us to try.
Dives in Misericordia, the second encyclical of Pope John Paul II, is available online here, and is copyright © 1980 – Libreria Editrice Vaticana