Setting the appropriate Gospel texts from Matthew and John, these passions include a narrator who tells the Biblical story in recitative (style of singing that resembles speech) while various soloists play different character roles in the story. All the while, the choir acts as the commenting disciples/crowd in the background. The choir and soloists are accompanied by a small ensemble.
Here is a video of Bach’s St. John Passion conducted by John Eliot Gardiner at the BBC Proms. Notice the size of the orchestra and choir. This most likely is a more modern interpretation of the work, as Bach would have likely used a small group of vocalists and instrumentalists.
Bach had a close relationship with God and I think this is apparent listening to works such as these. His worship and devotion to his creator is written in a way that moves the worshiper in the audience.
Have you ever seen a live performance of either the St. Matthew Passion or St. John Passion? If so, what did you think?