This well-known and beloved piece is one of Bach’s several interpretations of the classic German Advent hymn "Nun komm, der Heiden Heiland" (Hymn #54, “Savior of the nations, come!” in The Hymnal 1982 of the Episcopal Church).
Here is how I understand and tried to interpret this piece. . . .
One can justifiably assert that this hauntingly expressive work by J. S. Bach manifests a powerful degree of mysticism—not tranquil contemplative mysticism, but the sort that cries out in deep yearning for God’s arrival among us. To express that, a florid solo-melody boldly proclaims the believer’s anxious longing for the Savior’s coming, while the contrapuntal accompaniment—especially in the bass—generates the steady pulse of Christ’s approaching footsteps.