Matthew 5:4 (ESV) … “Blessed are those who mourn, for they shall be comforted.”
The second beatitude promises comfort to those who mourn. In the parallel saying in Luke 6:21, the contrast is expressed in terms of weeping and laughing. There the language recalls Psalm 126:5–6. Matthew’s version, however, uses the vocabulary of Isaiah 61:2–3, which speaks of comfort for those who mourn the plight of Israel following the exile.
This gives us some clues as to the grief of the mourners whom Jesus calls blessed. Their grief is not merely bereavement at the grave, nor is it limited to sorrow over personal sin. What they mourn is the condition of the world as it now exists—and all the loss and suffering they have experienced in this world. Like Simeon (cf. Luke 2:25), they are “looking forward to the consolation of Israel,” for a new age in which sorrow no longer reigns.
Such persons, Jesus says, will be comforted. Here the passive voice points to the action of God, who will transform the world and turn mourning into joy (cf. Isa. 49:13; 66:13; Jer. 31:13; 2 Cor. 1:4; Rev. 21:4). This “comforting” reversal of the present scheme of things is once again a matter of hope. It is a hope, however, which begins to be fulfilled as Jesus speaks. His assurance of comfort is itself a foretaste of the comfort yet to come. 
In another seeming contradiction in terms, Jesus explained that those who mourn are blessed. Jesus reminded his disciples that the prophet Isaiah had promised that the Messiah would “comfort all who mourn” (Isaiah 61:2 niv). Scholars differ on the exact nature of this mourning. Some say that Jesus was referring to the nation of Israel mourning for its sins; others interpret this more personally, explaining that it refers to those who mourn for their own sins or even for personal grief or oppression. Tied with the beatitude in verse 3, this means that humility (realization of one’s unworthiness before God) also requires sorrow for sins. Still other scholars see in the word mourning a picture of God’s people who suffer because of their faith in him.
Whether Jesus’ followers mourn for sin or in suffering, God’s promise is sure—they will be comforted. Only God can take away sorrow for sin; only God can forgive and erase it. Only God can give comfort to those who suffer for his sake because they know their reward in the kingdom. There he will “wipe away every tear from their eyes” (Revelation 7:17 niv). Jesus explained to his disciples that following him would not involve fame, popularity, and wealth. Instead, it could very well mean sorrow, mourning, and suffering. But they would always know that God would be their comfort.