Study this one hard! Sopranos have it easy for the most part because they are singing the chorale tune, but the rest of us have work to do! Listen, however, for small parts of the chorale tune in the other voice parts as well. Normally Bach introduces each line of the chorale tune in the Alto, Tenor, or Bass section accompanied by a counterpoint, then the sopranos sing the same obbligato line stretched out over several bars.
Listen how the music changes when the words are "Hallelujah, Hallelujah". After singing about how 'Christ lay in deaths grip,' the music speeds up (we wont speed up quite this much though) and becomes more dance-like when we offer God praise and thanksgiving through the word Hallelujah!