St. Isaac the Syrian : What is a merciful heart?

It is a heart on fire for the whole of creation, for humanity,for the birds, for the animals, for demons, and for all that exists.

By the recollection of them the eyes of a merciful person pour forth tears in abundance. By the strong and vehement mercy that grips such a person’s heart, and by such great compassion, the heart is humbled and one cannot bear to hear or to see any injury or slight sorrow in any in  creation. For this reason, such a person offers up tearful prayer continually even for irrational beasts, for the enemies of the truth, and for those who 

harm him, that they be protected and receive mercy… because of the great compassion that burns without measure in a heart that is in the likeness of God.