"Mourn" is the Greek word pentheo (Pres.part.). Generally it means, "to lament, to mourn for the dead. To grieve with a grief which so takes possession of the whole being that it cannot be hid" (Fritz Rienecker, The Linguistic Key to the Greek NT, 12). This is the strongest term used for sorrow. "It represents the deepest, most heart-felt grief." It carries "the idea of deep inner agony, which may or may not be expressed by outward weeping, wailing or lament" (John MacArthur, Matthew 1-7, 154). The mourning that Jesus is referring to here is not the mourning for the dead but mourning over sin. When you have the right attitude toward sin, which is "poor in spirit," then you mourn over it. Thomas Watson says, "Our mourning for sin must be so great as to exceed all other grief. Eli's mourning for the ark was such that it swallowed up the loss of his two children. Spiritual grief must preponderate (to outweigh) over all other. We should mourn more for sin than for the loss of friends or estate. We should endeavour to have our sorrow rise up to the same height and proportion as our sin does" (The Beatitudes, 71). It's important to understand that the "happiness or blessedness does not come in the mourning itself. Happiness comes with what God does in response to it, with the forgiveness that such mourning brings. Godly mourning brings God's forgiveness, which brings God's happiness" (MacArthur). Thomas Watson again says, "We must go through the valley of tears to paradise. Mourning were a sad and unpleasant subject to treat on, were it not that it has blessedness going before, and comfort coming after. Mourning is put here for repentance. It implies both sorrow, which is the cloud, and tears which are the rain distilling in this golden shower; God comes down to us" (The Beatitudes, 59).

There is an improper mourning. "This is the sorrow of those who are frustrated in fulfilling evil plans and lusts or who have misguided loyalties and affections" (MacArthur). Thomas Watson calls this "diabolical mourning...when a man mourns that he cannot satisfy his impure lust, this is like the devil, whose greatest torture is that he can be no more wicked" (The Beatitudes, 60).