It is frequently the task of musicologists and serious-minded performers, when dealing with so extensive a musical output as J. S. Bach's (not only an extensive output but also one produced before the advent of copyright laws or even reliable publication and whose works were thus subject to all sorts of changes and rewrites as they got transmitted down from one manuscript to another over the decades), to sort through all the many versions of a given piece that exist and to try, at the end of all this digging, to come up with what might reasonably be called the "authentic" version of that piece. Very often in the case of Bach and other high-profile composers, the task is impossible, but sometimes the evidence in favor of one version or another is strong enough for the myriad individually-minded scholars to reach a consensus and the catalog of works to be altered to reflect the new agreement. This is more or less what has happened over the years regarding Bach's organ chorale prelude on "Nun freut euch, lieben Christen gemein." The prelude, thought to have been composed sometime during Bach's years as court organist in Weimar (1708-1717), survives in two versions, sometimes cataloged as BWV 734 and BWV 734a. In the one version of the piece, the original Lutheran chorale melody (the cantus firmus, it is called) on which Bach's elaboration is based is played by the hands on the organ manuals; in the other version the cantus firmus is found in the organ pedals, with a few minor but, when dealing with so important a part of the repertoire as Bach, vital changes to the music of the other voices. Neither version survives in a truly authoritative source, but over time and for a variety of reasons, the version with the cantus firmus in the pedals has fallen somewhat into scholarly disfavor and more often than not been labeled "inauthentic," leaving the manual-only version of "Nun freut euch, lieben Christen gemein" as the sole heir to the heading BWV 734.
The melody of "Nun freut euch, lieben Christen gemein" is one of the earliest Lutheran hymn tunes, having been derived by Martin Luther himself in 1524 (the tune was also associated, from 1682 on, with the text "Es ist gewisslich an der Zeit," and one will occasionally find BWV 734 under that title). Bach puts this cantus firmus in the tenor voice, to be played by the inner fingers of the left hand while the bass moves along in steady eighth notes and the right hand indulges in a florid sixteenth-note obbligato whose opening tones subtly foreshadow, in outline, the first five or six notes of the tenor's cantus firmus melody. As in the original hymn, the first pair of phrases are repeated; the final three phrases make for one long push towards the final G major cadence, richly extended (in Bach's usual plagal/subdominant way) by the running bass and treble obbligato under the umbrella of the tenor's sustained G pedal tone.