Johann Sebastian Bach (1685-1750) composed these eighteen chorales near the end of his life.
The "J." on top of the heading means Jesu Juva (Jesus, help!). It stands as a motto over the Eighteen Chorales, just as it also embellishes other works.
For more about these chorales, especially the controversial "deathbed" 18th chorale, you may enjoy reading Larry Visser's analysis