According to the Catechism of the Catholic Church 1829, mercy is one the fruits of the theological virtue of charity (love). The theological virtue of charity is defined as "the theological virtue by which we love God above all things for his own sake, and our neighbor as ourselves for the ove of God." (CCC 1822) Hence we cannot understand mercy without understanding the theological virtue of charity. Mercy, being a fruit of charity is directed to one who has offended; a sign of compassion to such a person.
There is a directionality to mercy: conversion through forgiveness. It is an imitation of Christ. The disposition of mercy cannot be merited even if the reception of mercy does require a response. We are reminded by St Paul that Christ died for us when we were steeped in sin (Romans 5:8) He does this because He loves us. This does mean that God approved of, ignored, or enabled us to sin. No. He calls us from sin and gives a path home. In the Scriptures, we see God places a premium on mercy, as we see in Mathew 9:13: It is mercy I desire. It happens in the call of Matthew. Notice, that the exercise of mercy does not leave the person where they were. However the exercise of mercy can be uncomfortable and risks being rejected.
In the Church we talk about the corporal works of mercy AND the spiritual works of mercy. The corporal works of mercy are: Feeding the hungry, giving drink to the thirsty, clothing the naked, sheltering the homeless, comforting the sick, visiting the imprisoned, burying the dead. These are easily seen as acts of charity. They all require a person to look to the needs and goods of another. These are easy to identify. I will point out that the word 'worthy' appears nowhere (as in feeding the worthy hungry). To imitate Christ is to imitate His willingness to look out for the good of all.
The spiritual works of mercy, though, are a little more uncomfortable. They are :instructing the ignorant, counseling the doubtful, admonishing the sinner, comforting the sorrowful, bearing wrongs patiently, forgiving all injuries, praying for the living and the dead. The same love that motivates the corporal works of mercy motivate the spiritual works of mercy. It is these works that create the strife. The extremes fall into camps: one camp sees the law as a weapon to beat sinners with and the other turns a blind eye to sin and leaves the person there. Neither camp knows mercy. Where there is no mercy, there is no charity.
Mercy is not the approval of, condoning of, nor acceptance of sin. The tolerance police love to believe that this is the case. Toleration is meant for persons, not actions. A sin does not cease to be a sin because I love the person. In fact, if I love the person, I will not want to see them remain in that which harms them. Sometimes that means leaving the door open as the father does in the prodigal son parable. His son leaves of his own volition and must return of his own volition. He does not go chasing after his son, but once the son has returned he forgives the immense harm that his son has done to him. Mercy cannot force conversion nor remove the need for conversion; it is continually open to conversion. To condone sin and withhold engaging in the spiritual works of mercy is as bad as to have no mercy at all. Ezekiel 33:8 reminds us to leave the sinner in sin is to bring their death upon our own head. To do nothing is not an option.
On the other side we have the one without mercy. This is a dangerous and spiritually fatal place to be. If we look at the Sermon on the Mount in Matthew 6:14-15, immediately after Jesus gives us the Our Father, He says, "If you forgive others their transgressions, your heavenly Father will forgive you. But if you do not forgive others, neither will your Father forgive your transgressions"To withhold mercy is call upon God to withhold mercy. What a dangerous place we put ourselves in taking the attitude , "You sinned and and beyond my desire to forgive, you are going to hell." In such an attitude we consign our own soul to hell. Forgive us our trespasses as we forgive those who trespass against us? Sound familiar? Mercy reaches out to the sinner to pull them from the wreckage of their sin: not to leave them in the wreckage not condemn them for having the wreck. The balance is hard but necessary to strike. That people would resent that a sinner could be forgiven is the older brother in the parable of the Prodigal Son...it is they who now are ostracized from the familial bond of the Body oft Christ.
We show ourselves to be God's children when we seek to be merciful. St John reminds us that God is love. The Church reminds us that mercy is a fruit of love. Without mercy, we have no love and will cheat ourselves out of the kingdom of heaven. Whether that lack of mercy comes from failing to forgive or giving approval for the sinner to remain in sin is not relevant, it accomplishes the same end and endangers our souls in the process.
One of the things that Pope Francis asked for in this year of mercy for a greater use of and access to the sacrament of Reconciliation. It is the entrance way back into the Church after mortal sin has been engaged. We cannot ask our mortal sins be forgiven and still hold onto the sins against us. Have doubts? Read Matthew 18:21-35. Showing mercy the ultimate act of self preservation! Refusing mercy is the surest source of eternal damnation. Getting to judge people is not a perk of being a good Christians...it is a warning sign that something is going fatally wrong. we are to seek the eternal good of all, especially those who are acting against that eternal good. But charity must be the salve in which we wrap the medication. IT MUST!
I recognize that I am in need of much mercy from God. If I know this, I must equally realize that I must also be generous with mercy; not merely extending it to those I like, but to those who stand against absolutely everything I stand for as well. I must be willing to show mercy for those who have no mercy for me.