Most people tend to think of mercy in terms of withholding punishment. In contemporary English one “begs for mercy” or “throws themselves on the mercy of the court.” The person who wants mercy is admitting their guilt and hoping a judge shows them mercy and withholds their full deserved full punishment.
But both the noun (ἐλεήμων) and verb (ἐλεέω) refers to being merciful to others: “being concerned about people in their need” (BDAG), primarily by giving to the poor. The word can have the nuance of compassionate or sympathetic, but in the Hebrew Bible it often refers to actions which are compassionate.
This is the word often used to translate the word hesed in the Septuagint. The word is used often and is one of the key theological terms in the Hebrew Bible. Hesed is steadfast love, loving kindness, even covenant loyalty. As such, it is one of the key characteristics of God in the Hebrew Bible. In Exodus 34:6 and Numbers 14:18 God is “merciful and gracious, slow to anger, and abounding in steadfast love (hesed, LXX ἐλεήμων) and faithfulness.” God’s merciful character is the basis for many commands in the Law. For example, Exodus 22:6 the prohibition of taking a man’s cloak as a pledge is based on God’s merciful character.
A major difference between contemporary usage of the word mercy and the Hebrew Bible is that one does “an act of mercy.” In the LXX, Proverbs 28:22 contrasts the stingy man with a merciful (ἐλεήμων) person. In Acts 9:38 Dorcas is described as always doing good and helping the poor, in 10:4, Cornelius was praised for generously giving to the poor. Both of these are “acts of mercy.” The true disciple of Jesus imitates God’s merciful and generous character by meeting the needs of other people.
The promise in this beatitude is that the merciful will themselves be shown mercy. The verb is a future passive, often called a divine passive since the implied actor in the sentence of God. God himself will give mercy to the ones who do mercy. If doing mercy is meeting the physical needs of others, then this verse says God will meet the needs of those who help others. This is consistent with the previous beatitudes, the hungry will be filled, the mourners will be comforted, etc.
This should not to be dumbed down to sappy pop-culture karma or a Christian form of “what goes around comes around.” If it is God’s nature to help people in need then it is the nature of God’s people to do mercy towards those in need. It is possible western Christians think they are being merciful towards those in need when they give money to relief programs for the majority world, but does that fully engage this beatitude? How does the true disciple of Jesus “do mercy” to those who are in need?