The cantata is an early one, written while Bach was still at Weimar. (A second viola part would be unusual in later works; there's a joke to be made about violists here, but I figure they receive enough abuse as it is.) It is dispiriting, perhaps, to recall that he wrote it while still in his twenties. How is it that such a young man could produce a work of such perfection? The answer lies partly in Bach's immense genius, of course, but also in the culture that produced him. Musicians and theologians were cultivated then in the way we cultivate athletes today. We have an efficient system for recognizing athletic talent at a young age and encouraging it; Bach was the end result of generations of musicians teaching their children the art and livelihood of music. He grew up living and breathing harmony and counterpoint, Lutheran chorales, and instrumental technique, absorbing all musical styles he encountered. We will never produce another Bach, or Shakespeare, because our priorities as a culture as so different (read: much worse).
BWV 12 was first performed on April 22nd, 1714, which, as it happened that year, was the third Sunday after Easter, Jubilate. (I think these old names for Sundays in the church year are worth keeping.) The cantata, however, is not particularly jubilant. Consider its titular chorus:
Weinen, Klagen, Sorgen, Zagen Angst und Not Sind der Christen Tränenbrot, Die das Zeichen Jesu tragen. |     | Weeping, lamentation, worry, apprehension, anxiety and distress are the bread of tears of Christians who bear the mark of Jesus. |
It is not hyperbole to say that Bach's cantatas, taken as a group, represent the full range of human emotion. This is distinctly at odds with church music as it is commonly conceived nowadays. We have become unable to recognize grief and suffering as an integral part of the Christian life. Consider nearly every new piece of church music written in the last half-century: how many even attempt to deal with grief, let alone in a theologically and intellectually honest manner? (My complaint, here, is directed at the "new" "music" of Haugen, Haas, et al. that has wormed its way into Evangelical Lutheran Worship and other such hymnals. Let us not discuss the even-less-disciplined approach of "Praise" music.) We are afraid to recognize sadness because doing so would remind us that religion is about more than a constant endorphin high; seeking to stay on the heights at all times, we ignore the depths the psalmist was talking about. If the Christian religion is worth retaining, we must endeavor to regain the integrity of grief, honestly considered and addressed. An excellent way to begin this process is with our music.