Despite this disagreement about when we should celebrate Bach's birthday, what is generally agreed is that Bach was one of the greatest composers in history. Some would even argue that he was the greatest composer of all, but, obviously, that can be debated. What is not in doubt is the fact that not only was Bach a great musician, he was also a deeply devout Christian. Given that some of his most famous choral works, like the St. Matthew and St. John Passions, are based on stories from the New Testament, and that many of his organ works are based on Protestant chorales, this is perhaps not surprising. However, Bach was not only a composer of sacred works. For example, some of his best known works, like the Brandenburg Concertos and the Goldberg Variations, do not have an explicit connection with the Bible or Christian theology.
Nevertheless, Bach himself "made no real distinction between sacred and secular music. For instance, at the beginning of such a 'secular' work as his Little Organ Book, he wrote this dedication: 'To God alone the praise be given for what's herein to man's use written.' His Little Clavier Book, like so many of his compositions, was inscribed 'In the Name of Jesus.'" (Patrick Kavanaugh, Spiritual Lives of the Great Composers, p. 20). In inscribing his compositions "In the Name of Jesus," Bach was following the command of the Apostle Paul: "And whatever you do, whether in word or deed, do it all in the name of the Lord Jesus, giving thanks to God the Father through him" (Colossians 3:23, NIV). Moreover, according to Patrick Kavanaugh, "Bach resoundingly echoed the convictions of Luther, claiming that 'Music's only purpose should be for the glory of God and the recreation of the human spirit'" (Spiritual Lives of the Great Composers, p. 19; citing Robert W. S. Mendl in The Divine Quest in Music).
In short, Bach saw his work, whether explicitly "religious" or not, as being a means of bringing glory to God. We might do well to follow his example in our own lives--even though we are unlikely to attain to Bach's level of achievement.
Image of a statue of J.S. Bach in Leipzig, Germany from Wikimedia Commons