This cantata is listed first among Bach's surviving, but that does not mean it was the first he completed. In fact, this cantata was written in March 1725 for the Feast of the Annunciation. (Bach began writing cantatas as early as 1706.) This is one of the Marian feasts of the church, meaning that it focuses on Mary, the mother of Jesus. The Annunciation is when the Archangel Gabriel came down from heaven to announce to Mary that God had chosen her to give birth to His Son.
The text is based on the Biblical story of the Annunciation, but it is a poetic adaptation of that story, written by Philipp Nicolai, who provided text and music for a number of Lutheran chorales. Bach was certainly familiar with his work - in fact, Nicolai was also responsible for the chorale which forms the basis for Cantata 140.
The text focuses on Mary, not in every movement, but varies with both direct and indirect references in several movements. For example, the second movement, a recitative, begins with the words "thou very Son of God and Mary born." The third movement, an aria, paraphrases some of Mary's words of joy quoted in the Bible which describe her joy when she realizes what her Lord has asked of her: "My spirit with rapture is ardently burning, unceasingly yearning, to know all the joys that await me above." To emphasize the connection between these words and Mary's feelings, Bach uses a soprano to sing the aria. This aria is scored for soprano, oboe da caccia, and continuo. Because it employs only three independent parts, it has a very intimate feeling, much like the personal intimate feelings Mary must have felt upon receiving the visit from Gabriel.
This cantata combines elements of a number of different genres. First is the Lutheran chorale. Bach often uses chorales in his cantatas. This one uses the same melody for the first and last pieces. Typical of Bach, the first movement is a chorale fantasia, which means that the sopranos of the choir sing the chorale melody very simply, using long note values, while the other voices in the choir and the instruments of the orchestra fashion a complicated tapestry around and beneath that melody. This creates a very polyphonic texture, in which many independent and active parts are performed simultaneously.
The final movement is a more-or-less traditional chorale setting. That means the sopranos of the choir, supported by the highest-sounding instruments of the orchestra, have the melody, and the other parts provide chordal accompaniment. (This is a homophonic texture.) What is unusual here is that Bach writes a Horn II part which is independent from the rest of the texture. This independent line harmonizes well with the other parts, at the same time providing a more interesting line within the otherwise simple texture.
The second genre employed here is the concerto grosso. This is a concerto in which a group of soloists (called the "concertino"), rather than a single soloist, is featured in displays of virtuosity. The use of the concerto grosso is obvious just by looking at the first page of the score, where Bach indicates parts for "violino concertante I and II" and parts for "violino ripieno I and II" (meaning the rest of the orchestra).
These virtuosic violin passages are present only in the first movement, but the first movement comprises more than half of the music in the cantata.