The most highly regarded poet in the English language compared mercy with rain:

"The quality of mercy is not strain'd.
It droppeth as the gentle rain from heaven
Upon the place beneath. It is twice blest -
It blesseth him that gives and him that takes."  --Shakespeare

Mercy is a mystery, perhaps.  It's not a word most of us use in everyday conversation, unless we have the habit of saying, "Mercy!" instead of cussing.

Instead of trying to define the word, I'd rather point to the way mercy functions.  The gift of mercy spreads the kingdom of heaven; it is the rain that waters the seed of the Kingdom of Heaven and, with regular and gentle repetition, eventually leads to a bountiful harvest.  Jesus gives a good description of the cause and effect of mercy:

Luke 6:37-38

36 "Be merciful, just as your Father is merciful.  37 Do not judge, and you will not be judged. Do not condemn, and you will not be condemned. Forgive, and you will be forgiven. 38 Give, and it will be given to you. A good measure, pressed down, shaken together and running over, will be poured into your lap. For with the measure you use, it will be measured to you.”

In the parable of the ungrateful servant, the King--God--does not ask us to pay the unbearable debt we owe him; he excuses it.  He acts toward us with mercy.  A lot of people would describe this as the gospel: the good news.  Yet Jesus stipulates:  God shows mercy toward the merciful.  The unmerciful, however, will not be excused their debt.

If I were a theologian, this sort of parable could drive me nuts.  What happened to our good news?  Do all who are thirsty get to drink freely from the waters of life, or don't we?  Is showing mercy a work of our own, and if it is, will works actually save us?  Was Luther barking up the wrong tree entirely?  How does God get away with failing to show mercy, yet hold everyone else up to the standard?  Does the gospel of grace fall apart on the point of mercy, received and given?

When I was young, my father would lecture me for not understanding the value of money.  He was brought up in the Depression and I wasn't, so I'm sure I still don't understand money the way he did. 

What was worse, perhaps, was that I did not understand the value of mercy.  When I began to appreciate the value of mercy, I understood the obligation that accepting the free gift of forgiveness placed on my heart. I had no choice but to forgive the injustices others had enacted on me.  I learned to step aside and let God act through me and do what he does best: I consciously forgot the obligations of others and I passed the mercy along.   Sometimes I still fail, but everytime mercy happens through me, I realize that my bond to the giver of the gift is more powerful than any debt owed to me.

When I began to look at God's mercy to us as a free gift, and the mercy we extend to others as an outward sign that we have recieved that gift, it all fell together.  Mercy is not a work that saves us, it's sign of a well-watered garden.  It shows we've been blessed with showers of grace, we've been immersed in forgiveness, and we are drinking the living water of God's presence in our lives.

If I have been shown mercy and my own debt has been forgiven but I demand every last penny from my friend, it's as though I went to the fountain, I talked about the fountain, I had the water in the fountain analyzed, I read the report about the water in the fountain, I rejoiced in existence of the fountain, perhaps I even threw a party to celebrate the fountain and served up Coke and Sprite from a fully stocked open bar, but I never really drank from the fountain.  And if I insist on talking about the living water and never drinking it, then I must not be thirsty.  Not God nor anybody else can make me drink the living water when I don't choose to. 

If I do at last drink the living water, everyone will know it.  Mercy will begin to flow freely from me out to people around me, and never stop.

The kingdom of heaven is not built on lip service, selfishness, and opportunism but on experiencing the presence of God in spirit and in truth.  When we embody God's grace with our own actions, we allow others to experience God's presence in their lives and we take part in spreading the Kingdom of heaven. 

Like God's free gift of rain on the just and the unjust, mercy increases the harvest.  Let mercy lead.

"And I'm drawing these words from a well that's so nearly dry,
A thirsty man would surely die in need of a drink. 
And where once there was life, there's a graveyard of memories,
The immobile energies of paper--paper and ink."  

--Windows to Wonderland, Rich Mullins 1975   

Rich Mullins' second year in Bible College, he was struggling with rationalism--the idea that everything we can learn about God comes through our minds and our direct senses, through analyzing words on a page.  But Richard was constantly thirsty for more than words.  Eventually, he came to believe that God wants us to worship with our lives, not just our words.  This is true worship, the living water he'd thirsted for.   "Windows to Wonderland" asks the questions that Richard soon answered in his original version of "If I Stand."   

Living Waters: When will our thirst be quenched? 

In Israel, times and places of worship were strictly structured; guidelines of purification also needed to be met before worshippers could experience the presence of God.  If the desire for God's presence is a thirst, when will it be quenched?  The days of temple worship are over.  Do we have to live out our lives dry, thirsty, waiting to meet God in heaven at last?  Jesus spread the good news that the drought is over--now.   "The day is soon coming, and is now here, that you will worship. . ."  The Greek word for worship conveys the idea of "kissing the hand towards,"  significantly, not an abstract state of mind, but an action.  Words may express worship, but verbal worship fails when our words and our actions do not match.  "you will worship God in Spirit and in truth."  God cannot be confined to words on a page.   God is worshipped in Spirit, and in the truth of our lives.         

In the Hebrew tradition of purification, there were times when living water (running water from a stream, spring or ritual pool) was indispensible.  Before entering the temple, before the bride's wedding ceremony, after any ritual uncleanness, upon initiation into their faith, men and women of the nation of Israel used living water for the cleansing mikveh, a total immersion in living water.   

The Thirst: 
Based on John 4:1-28 

Jesus' disciples had been initiating so many followers that the Pharisees started to hear about it.  So Jesus and his disciples left the living waters of Judea and moved on to Galilee.  

Jesus had to go through Samaria. So he came to a town called Sychar, to Jacob’s well in Samaria. There Jesus sat to rest in the heat of the day.    When a Samaritan woman came to draw water, Jesus said to her, “Will you give me a drink?”  

Not with her ears, but with her heart she heard him say, "You are a human being, just like I am."    

Because he treated her like an equal, the woman spoke boldly:  “You are a Jew and I am a Samaritan woman. How can you ask me for a drink?”   Jesus answered her, “If you knew the gift of God and who it is that asks you for a drink, you would have asked him and he would have given you living water.”  

But she heard him say, "I know you are a woman and must cleanse yourself with living water in order to worship.  I know it must be hard to want God so much and to have to work so hard to come near him.  I can make it all so much easier."    

“Sir,” she came back, “you have nothing to draw with and the well is deep. Where can you get this living water?"   

And in her mind she thought, "Just how exceptional is this this man?  He thinks he can bring running water into a dry land!"   

And out loud she asked, "Are you greater than our father Jacob, who gave us the well and drank from it himself, as did also his sons and his livestock?”   Jesus answered, “Everyone who drinks this water will be thirsty again, but whoever drinks the water I give them will never thirst. Indeed, the water I give them will become in them a spring of water welling up to eternal life.”  

Her heart began to awaken.  She remembered the psalm, "Like a parched land, my soul thirsts for you. . ."   

She  said to him, “Sir, give me this water so that I won’t get thirsty and have to keep coming here to draw water.”  

He told her, “Go, call your husband and come back.”  

“I have no husband,” she replied.  

Jesus said to her, “You are right when you say you have no husband. The fact is, you have had five husbands, and the man you now have is not your husband. What you have just said is quite true.”  

Her heart and her life matched her thoughts, and instantly truth fell from her lips.  “Sir,” the woman said, “I can see that you are a prophet."  

Her heart pounded as she asked herself, "This man knows the truth.  But what about me?  What if I have been wrong all my life?  And now that I meet him, will the one who has the right answers tell me that my worship has been for nothing?"  

Out loud she said, "Our ancestors worshipped on this mountain, but you Jews claim that the place where we must worship is in Jerusalem.”  

“Woman,” Jesus replied, “believe me, a time is coming and has now come when you will worship the Father neither on this mountain nor in Jerusalem, but in Spirit and in truth.  These are the kind of worshipers the Father seeks.  God is spirit: he is to be worshipped in Spirit and in truth.”  

The woman said, “I know that Messiah is coming. When he comes, he will explain everything to us.”

In her heart she whispered, "Are you . . . you the one?"  The man was wise, kind, fair, and a prophet.  She knew she could trust his word.  Her throat went dry while she waited for his answer.

Then Jesus declared, “I, the one speaking to you—I am he.”