The text comes from the Beatitudes (Matthew 5):
Blessed are the pure in heart, for they will see God. Blessed are the peacemakers, for they will be called children of God. Blessed are those who are persecuted because of righteousness, for theirs is the kingdom of heaven.
Here's the chant score, from the Brazilian Benedictines:
Here's a lovely motet based in this text, by one Zieleński Mikołaj:
Wikipedia has this about him:
Mikołaj Zieleński (Zelenscius, birth and death dates unknown) was a Polish composer.
Zieleński's only known surviving works are two 1611 liturgical cycles of polychoral works, the Offertoria/Communes totius anni. These were dedicated to the Archbishop of Gniezno, Wojciech Baranowski. The sets consist of large-scale double- and triple-choir antiphons, as well as some monodic works typical of the Seconda pratica style of early Monteverdi. Zieleński's music is the first known Polish music set in the style of the Baroque.
That says, if I'm not mistaken, that this composer wrote polyphonic versions of all the Offertories and Communios for the whole church year! And that's all we've got of his. That is pretty amazing, really.
I expected to find many more polyphonic versions of this chant! But there don't seem to be many; William Byrd wrote one, but there's no online version of it that I could find.
The Orthodox, however, seem to be much bigger on Beatitude music! Here's a gorgeous (Russian, it says) version, for instance (embedding has been disabled, so that's a link to YouTube).
Here's another nice version, "The eight beatitudes," in Georgian Orthodox chant:
Here's a terrific Arvo Pärt version, sung in English:
There are lots of modern praise versions of these, too! I suspect - although I don't know for sure (maybe somebody reading does?) - that this song, "Osiem blogoslawienstw" ("Eight Blessings") sung in Polish, is a version of the Beatitudes as well. In any case, it's quite lovely, so here it is:
Its' interesting that the West did not, apparently, do as much with the Beatitudes in music, isn't it? They would seem to make up a perfect text for singing. Here they are, in full, from the Gospel reading for this Sunday:
When Jesus saw the crowds, he went up the mountain; and after he sat down, his disciples came to him. Then he began to speak, and taught them, saying:
"Blessed are the poor in spirit, for theirs is the kingdom of heaven.
"Blessed are those who mourn, for they will be comforted.
"Blessed are the meek, for they will inherit the earth.
"Blessed are those who hunger and thirst for righteousness, for they will be filled.
"Blessed are the merciful, for they will receive mercy.
"Blessed are the pure in heart, for they will see God.
"Blessed are the peacemakers, for they will be called children of God.
"Blessed are those who are persecuted for righteousness' sake, for theirs is the kingdom of heaven.
"Blessed are you when people revile you and persecute you and utter all kinds of evil against you falsely on my account. Rejoice and be glad, for your reward is great in heaven, for in the same way they persecuted the prophets who were before you.
The Collect for the day is this one:
Almighty and everlasting God, you govern all things both in heaven and on earth: Mercifully hear the supplications of your people, and in our time grant us your peace; through Jesus Christ our Lord, who lives and reigns with you and the Holy Spirit, one God, for ever and ever. Amen.
Here's Fra Angelico's "Sermon on the Mount," one of the frescoes he painted on the cell walls of the San Marco Convent in Florence: