As with the first beatitude, the second focuses more on the outward circumstances, rather than the inner disposition, of its subjects. Just as “blessed are the poor in spirit” describes a willingness to suffer material loss for the sake of the kingdom rather than a sense of inner, spiritual bankruptcy, so “blessed are those who mourn” has more to do with lamenting the seeming suspension of God’s justice in this world than it does with feeling sad in our hearts.
Comparing Matt. 6:4 with the corresponding beatitude in Luke’s gospel is helpful here. He doesn’t say “blessed are you who weep in your hearts,” but “blessed are you who weep now, for you shall laugh” (6:21). The contrast, then, is not between the inward and the outward, but between the present and the future.
I mean, anyone can mourn—it’s as easy as reading the morning paper, getting irritated, and resolving to do something about the direction this world is heading. The hard part of the beatitude is the second half (the part that postpones our comfort to a future time). In a word, what we do with our mourning reveals where our heart’s treasure lies. And if this is true, we may justly shudder as we consider the this-worldly nature of the hope that characterizes much the evangelical and Reformed church.
What makes this beatitude especially astonishing is how different its ethos is from that which was espoused in the Old Covenant law. Even a cursory reading of the second half of Deuteronomy 28 will amply demonstrate that, according to Moses, “Cursed are those who mourn.” Israel, as an earthly type of heavenly glory, was expected to exhibit military might and material abundance as tokens of God's favor, and any failure to prosper was indicative of Yahweh’s displeasure. Jesus, it seems, overturns that principle altogether, and Paul, echoing him, actually appeals to his own experience of what used to be considered covenant curses in order to validate his own apostleship.
From a human perspective, it’s no wonder these men were killed.