This Sunday is the 500th anniversary of Martin Luther’s famous posting of his 99 theses on the door of the church in Wittenburg, considered the beginning of the Reformation. To mark this event, the choir this Sunday will sing the first movement of J. S. Bach’s cantata 80, a setting of Luther’s hymn, A Mighty Fortress Is Our God. In this magnificent piece, Bach takes each phrase of the chorale in turn, weaving contrapuntal lines derived from that phrase of the melody, before finally presenting the melody in long, plain notes in the bass (in our case, the organ pedals). The service will conclude the way Bach’s cantata does, with Bach’s ornate harmonization of the last verse of the hymn. In contrast, the second anthem is a setting of Psalm 146 from the 1565 Scottish psalter, the first complete psalm book and the first music of any kind to be printed in Scotland. While psalms had been part of the musical repertoire of the church for centuries, the Reformation gave them a new role, as the entire congregation (instead of a choir) began to sing the psalms in their own language (not Latin), set as rhymed poetry. While the choir will sing Psalm 146 alone, the congregation as a whole sings Psalm 90, in the paraphrase version of Isaac Watts (Our God, Our Help in Ages Past). The third hymn this week is I Greet Thee, Who My Sure Redeemer Art, a text attributed to John Calvin.