"Blessed are you who are poor ... woe to you who are rich."
"Blessed are you who are hungry now ... woe to you who are full now."
"Blessed are you who weep now ... woe to you who are laughing now."
"Blessed are you when people hate you ... woe to you when all speak well of you."
The Gospel of Luke is easy to love. In social justice-oriented contexts, Luke's is the go-to gospel because it so clearly portrays a Jesus who is deeply concerned about the social and material circumstances of the poor and the marginalized. It's the natural source for a Christian theological call for justice (assuming we're ignoring all the Hebrew Bible prophets, which as Christians we probably are. Thanks, supersessionism!). In Luke's rendition, the Beatitudes are a pretty uncompromising set of eschatological reversals, "comforting the afflicted and afflicting the comfortable," as we bleeding-hearts like to say.
Matthew, on the other hand, is a little more difficult. I've never heard anyone say that Matthew was their favorite gospel. Matthew is fussy and literalist to the point of being nonsensical, as when he tries to fulfil a Hebrew Bible prophecy by having Jesus enter Jerusalem riding two donkeys simultaneously because he's never heard of hendiadys. And look at what Matthew does with the first Beatitude:
"Blessed are the poor in spirit, for theirs is the kingdom of heaven."
For starters, note that Luke's Jesus is addressing the poor directly (as well as the rich, whom Matthew doesn't mention), whereas Matthew talks about them as though they are absent or even abstract. The most glaring difference, though, is that Luke's "poor" have become for Matthew "poor in spirit." It seems like a pretty clear-cut instance of Matthew softening the call for justice in order to avoid upsetting the wealthy and to accommodate the status quo.
I think there is potential for a more generous and more interesting reading of Matthew 5:3, though, and it arises from the question: what the hell does "poor in spirit" mean anyway?
Historical-critically, or with respect to the author's intentions, your guess is as good as mine. The reading I'm proposing has nothing to do with source criticism, the historical context in which Matthew's Gospel was written, or the nuances of the Greek terminology. It's just a reading I think is interesting, challenging, and kind of cool.
Luke's Jesus says: "Blessed are you who are poor, for yours is the kingdom of God." It's a promise of straightforward reversal: those who currently lack resources and comfort will receive abundance
Matthew's Jesus: "Blessed are the poor in spirit, for theirs is the kingdom of heaven." If this is also a reversal, Matthew is clearly not using "kingdom of heaven" in quite the same way Luke is using "kingdom of God." Neither the reward nor the affliction are material in nature.
What does "poor in spirit" mean? I suggest that we can read it quite simply, as the reverse of "the kingdom of heaven": the poor in spirit are those who lack a certain sensibility, which for want of a better term I call "religious." Not those who do not believe in God, who have no use for that hypothesis -- it's perfectly possible to be religious without believing in God -- but those who have no sense for, well, whatever you want to call it: the transcendent, the liturgical, the impossible, wonder, the perhaps. Something precarious, something humbling, something greater than oneself. The people with the shallowest, most mean-spirited view of what humanity can be (I'm sure we can all think of examples, including some who call themselves religious and believe in God) are blessed. Theirs is the kingdom of heaven.
This is no way replaces Luke's vision of material abundance for the materially poor, but it's a thought-provoking supplement. For someone like me, steeped in leftist academia and liberation theology, "blessed are the poor" is normative, but "blessed are the small-minded" is a real challenge.