3 Attend to the third lesson, “Blessed are they that mourn, for they shall be comforted.” The work consists in mourning, the reward in consolation; for they who mourn in a carnal sort, what consolations have they? Miserable consolations, objects rather of fear. There the mourner is comforted by things which make him fear lest he have to mourn again. For instance, the death of a son causes the father sorrow, and the birth of a son joy. The one he has carried out to his burial, the other he has brought into the world; in the former is occasion of sadness, in the latter of fear: and so in neither is there consolation. That therefore will be the true consolation, wherein shall be given that which may not be lost, so that they may rejoice for their after consolation, who mourn that they are in exile now.
Source: Sermons on Selected Lessons of the New Testament (New Advent)