Ein feste Burg ist unser Gott, English translation: A Mighty Fortress is by Martin Luther. It is the theme song for the Reformation. It is not generally considered 'gospel'.

There are several English translations. Cyberhymnal has 2 of them.

The orginal tune was very rhythmic but Bach or someone else from his period arranged it in an isometric version that most people know.

You can read more at History of Hymns