Cantata BWV 121, composed 1724 in Leipzig, is based upon the Luther chorale "Christum wir sollen loben schon.” Here is a Luther work treated in the respectful archaic manner. As with other Luther arrangements of Latin sources, the melody is very irregular in form, four phrases of irregular length. The second and third phrases are similar and have only one (but an important one) note difference, at the beginning. The last phrase also has similarities to the 2nd and third phrases. Perhaps the most interesting characteristic of the melody is that each phrase is longer than the last so that by the end the last phrase is almost twice as long as the first. There is another melodic peculiarity. The 2nd degree of the scale (the tune is in the Phrygian mode) is sometimes sharped and sometimes not. This ambiguity, which is different in every version of the chorale, gives rise to interesting harmonic variants. Bach uses it both as tone painting, as in the 2nd phrase that refers to the “pure maiden.” and allows the resultant harmony to lead the music in a different direction.
The opening cantata movement is somewhat different in texture than Bach’s other motet-style movements. Although each phrase begins with the lower voices entering imitatively, the figuration soon becomes dominated by a figure first introduced by the continuo. The alto, tenor and bass voices are dominated by variants of this pattern throughout the movement. The soprano, singing the chorale melody in long notes, dominates the texture even more than usual. The melody itself is probably as far from tonal as any melody Bach used, so that the amount of figuration needed to make it fit into a tonal mold is more than usual.
The tenor aria is marvelously off-kilter. The wonder at Jesus’ birth and the odd declamation of the 2nd line of text produces three bar phrases at the beginning which are only later molded into more conventional 2 and 4 bar phrases. In addition the mannerist setting of the text “O du von Gott erhöhte Kreatur, Bègreife nicht, nein, nein, betundre nur” with the weight on the first unaccented syllable further distorts the shape. The whole effect is of excitement and confusion. Imagine the shepherds at the cradle. The alto recitative follows up on the theme of confusion and wonder. It includes at the most important moment in the text one of the most hair-raising modulations in all of Bach. The change on the word “Art” not only colors the word but also achieves the important harmonic shift for the whole cantata.
The rather surprising introduction of the image of John jumping in his mother’s belly is not a part of the story one associates with this day. It gives us, however, a wonderful aria from Bach with the most marvelous orchestral texture illustrating the movement inside Elizabeth. There is also a sense that Bach is so glad to be working in a purely tonal context that he lets loose with all of the richest counterpoint and harmony that he has in his arsenal. The opening motive always appears in tight imitation, as if the movement reverberated through Elizabeth’s body. The opening tutti is quite long but so refulgent and full of ideas that we don’t miss the voice. The somewhat surprising modulation to E Minor for the B section not only refers to the harmony of the chorale, but also gives the words a properly mysterious context. When the opening theme appears in this section, it is ghostly and without imitation, like a distant memory. The full da capo is welcome and gives the aria its weight as the moral center of the cantata. As is so often the case, Bach gives the voice of the child soprano the last word, with an exultant yelp up to high b on the word ”Jauchzend.” Bach eliminates the tonal ambiguity that made such a difference in his opening chorus in his final chorale setting. It is as if the very resolution of the cantata depended upon it.