Der du bist drie in Einigkeit

Johann Michael Bach's prelude to "Der du bist drei in Einigkeit" is a well written four part piece. The tonal development is a bit strange. The piece start with a G major chord and ends with and A major chord. In between the music seems to hesitate between G major and e minor so the ending on a comes a bit as a surprise.

Score available here: http://partitura.org/index.php/johann-michael-bach-der-du-bist-drei-in-einigkeit