“A Christian is something before he does something.”  Martyn Lloyd Jones, Studies in the Sermon on the Mount (Grand Rapids: Eerdmans, 1959), 1:96.
That succinct statement sums up the point Christ made in the Beatitudes—that true righteousness is not determined by external works, but by the nature of the heart. We don’t necessarily think of the Beatitudes as a confrontational message. But as John MacArthur explains, that’s precisely how Christ intended it.
Jesus bypassed all the supposed credits [the Pharisees] had mounted to their own cause and went straight to the heart of the matter. Christ always puts the emphasis on the inside. He is not unconcerned with outward action, but only as it is produced by proper motivation. . . .
Living as a Christian means there is to be no veneer, no facade. Christianity is something that happens to us at the very center of our being, and from there it flows out to the activities of life. God has never been interested in only the blood of bulls and goats. He has never been interested in any superficial spiritual activity unless the heart is right. (See Amos 5:21–24.)  John MacArthur, The Only Way to Happiness (Chicago: Moody Publishers, 1998), 128-129.
The first four Beatitudes (which we discussed here and here) focus on internal principles about how we stand before God. The second set of four are still internal principles, but they deal with how we relate to others. Think of them as the fruit of their predecessors—if we are poor in spirit (humble), if we mourn over our sinfulness, if we’re gentle and meek, and if we hunger and thirst for God’s righteousness, we will act accordingly in regard to one another.
In Matthew 5:7, Christ describes the first facet of our interpersonal righteousness: “Blessed are the merciful, for they shall receive mercy.”
What Is Mercy?
We tend to equate mercy with God’s forgiveness of our sin, but as John MacArthur writes, the term encompasses much more.
It goes beyond compassion. It goes beyond sympathy. It means sympathy and compassion in action toward anyone in need. When our Lord talks about it here, the real eliamosuna is not the weak sympathy that carnal selfishness feels but never does anything about. It is not that false mercy that indulges its own flesh in salving of conscience by giving tokenism. It is not the silent, passive pity that never seems to help in a tangible way. It is genuine compassion with a pure, unselfish motive that reaches out to help. . . .
Mercy is seeing a man without food and giving him food. Mercy is seeing a person begging for love and giving him love. Mercy is seeing someone lonely and giving him company. Mercy is meeting the need, not just feeling it.  The Only Way to Happiness, 133-134.
That’s not to say that mercy and forgiveness are unrelated. As John explains,
Forgiveness comes from the fountain of mercy. We cannot think of mercy without its expression in forgiveness, and we cannot think of forgiveness without its source, mercy. But forgiveness is not the only expression of mercy. We cannot narrow mercy.  The Only Way to Happiness, 135.
In fact, the best way to understand mercy is through how it relates to other key facets of God’s character.
We said that forgiveness flows out of mercy. What does mercy flow out of? Love. Why has God been merciful? “But God, being rich in mercy, because of His great love with which He loved us” (Ephesians 2:4). Do you see the sequence? God loves and love is merciful, and mercy is forgiving, among many other things. . . .
What about mercy and grace? . . . The term mercy and all its derivatives always presuppose problems. It deals with the pain and the misery and distress. But grace deals with the sin itself. Mercy deals with the symptoms; grace deals with the problems. Mercy offers relief from punishment; grace offers pardon for the crime. First comes grace. Grace removes the sin. Then mercy eliminates the punishment.  The Only Way to Happiness, 136-137.
In the economy of God’s love, grace and mercy are two sides of the same coin. Mercy expresses His pity for fallen sinners; grace delivers His pardon for their sin. Mercy rescues us from hell; grace grants us a way into heaven. Mercy brings relief; grace brings blessing. We should be perpetually grateful for both sides of God’s love, and for the richness with which He bestows them upon us.
How Do We Show Mercy?
Now that we know what divine mercy truly is, how do we display and express it to one another? What does mercy look like in our everyday life?
John MacArthur explains that it shows up in our relationships, particularly in the way our relationships differ from the consistent patterns of the world. “Mercy never holds a grudge, never retaliates, never takes vengeance, never flaunts somebody’s weakness, never makes something of someone’s failure, never recites a sin.”  The Only Way to Happiness, 142. In short, mercy withholds the kind of abusive, manipulative, vengeful, and selfish ways people frequently treat one another. In fact, our relationships ought to be marked by a consistent pattern of mercy for others’ flaws and faults.
Moreover, we show mercy to one another in the way we meet each other’s spiritual needs—specifically through pitying, prodding, praying, and preaching. As John explains,
I hear Stephen saying as they cast the stones and crushed out his life, “Lord, do not hold this sin against them!” (Acts 7:60). He was pitying their souls. You and I must look at the lost with pity, not lording it over them or thinking ourselves better.
Next, we can prod. Second Timothy 2:25 tells us, “With gentleness correcting those who are in opposition, if perhaps God may grant them repentance leading to the knowledge of the truth.” Prodding means to confront people about their sin in order that God might give them forgiveness. They have got to hear the gospel.
Paul wrote Titus, “Reprove them so severely that they may be sound in the faith.” (Titus 1:13). I can care for the soul of a sinner by rebuking him to his face. Such an act is not unloving. In Jude 23 it says that there are some people whom you have to save with fear, “snatching them out of the fire.” That’s not hatred or cruelty; that’s love.
Mercy prods. There has to be the confrontation about sin before there can ever be the realization of sinfulness.
Next we can pray. Prayer for the souls of those without God is an act of mercy. Do we pray for the lost? Do we pray for our neighbors? Do we pray for Christians who are walking in disobedience? Our prayer is an act of mercy, for it releases God’s blessing.
Finally, we can preach. I believe preaching the gospel is the most necessary and merciful thing you can do for the lost soul.  The Only Way to Happiness, 143-144.
We can’t settle for lesser mercies in our relationships with the world. If we’re not faithfully pointing men and women to the gospel and the forgiveness of their sins, we’re withholding the greatest mercy imaginable.
Getting back to the blessing of Matthew 5:7, what can we expect as the result of showing such mercy to others? Christ says the merciful “shall receive mercy.” That’s not, as some have tried to claim, a promise of reciprocal kindness, peace, and harmony between man. Christ isn’t guaranteeing us that people will treat us the way we treat them. Instead, He’s explaining that the merciful will receive mercy from God.
Here’s how John MacArthur describes the cycle of mercy represented in this Beatitude.
The one who has received mercy will be merciful. The one who has received forgiveness will be forgiving. If you are a merciful person, you give evidence of being God’s child; so every time you sin, God forgives. Every time you have a need, He meets it. He takes care of you. He just pours mercy upon mercy upon mercy to those who show mercy, because they have received it from the merciful God.  The Only Way to Happiness, 145.