What is a Merciful Heart?

If following Jesus isn't making you more merciful, it might not be Jesus you are following.

Jesus said, "But go and learn what this means: 'I desire mercy, not sacrifice.' For I have not come to call the righteous, but sinners."
- Matthew 9:13
(He was quoting the Old Testament prophet, Hosea - Hosea 6:6).

Here is something from Isaac of Ninevah (died c. 700 AD), also known as 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 her or him, that they be protected and receive mercy. And in like manner such a person prays for the family of reptiles because of the great compassion that burns without measure in a heart that is in the likeness of God.

The person who is genuinely charitable not only gives charity out of his own possessions, but gladly tolerates injustice from others and forgives them. Whoever lays down his soul for his brother acts generously, rather than the person who demonstrates his generosity by his gifts.

God is not One who requites evil, but who sets evil right.

Paradise is the love of God, wherein is the enjoyment of all blessedness.

The person who lives in love reaps the fruit of life from God, and while yet in this world, even now breathes the air of the resurrection.

In love did God bring the world into existence; in love is God going to bring it to that wondrous transformed state, and in love will the world be swallowed up in the great mystery of the One who has performed all these things; in love will the whole course of the governance of creation be finally comprised.

(Taken from Glory to God in All Things)