This work is one of Bach's greatest creations; and tomorrow is Maundy Thursday, when according to the New Testament record, the Last Supper took place.
Those who are familiar with the story know that a lot was going on that night. When Bach wrote the great opening chorus of the Passion, this was one thing that he managed to put into it: the sense that a lot is going on. This was for good reason; by the end of Friday, Jesus was dead, the disciples had gone into hiding, Jesus' mother and the other Mary had fled the city, and few of them knew what was going on.
Musically, a huge amount of planning had taken place. As I've written before, there are two entire orchestras---modest-sized---spatially separated; two organs, and two choirs; and a third choir consisting only of trebles. That's what Bach wanted; whether it was performed this way in his lifetime, we don't know. (We do know that Mendelssohn staged a festival version of the Passion in 1832, which kicked off a Bach revival.)
If you're new to this work, and you get an uncomfortable sense of restlessness and foreboding, that's intentional.
In addition, the text of that first chorus does not present any part of the story; it only invites 'The Daughters of Zion' to come and mourn. I only know that this group of women---or this woman---is a symbolic representation of the Hebrew Nation.
While the daughters are mourning, the third choir, the Ripieno Trebles, are quietly intoning a chorale: Oh lamb of god unsullied. Altogether [this opening chorus] is an incredible achievement, that batters the listener for some 8 minutes. I recommend the performance conducted by Wolfgang Gonnenwein, of around the 1980s.
In certain parts of the world, at certain times, on Good Friday they have a three hour service. This was the case when I was growing up; it was a 3- hour meditation, but kids were permitted to leave after one hour, provided they kept perfectly quiet in the churchyard. It appears that this choral St Matthew Passion is in the same tradition, to force the congregation to engage with the horrors of those hours. It was considered to difficult for one minister to lead the entire meditation (When I was a kid), so they had a tag- team of three clergymen lead it in rotation.
In Bach's tradition, the Passions took the action through the crucifixion, to the point where the body is left in the grave. And that's what we hear in this work. The multiple choirs take leave of Jesus, each part in turn.