Although this hymn is largely, laudably based on Psalm 47, it is also closely associated with the Protestant Reformation.

On a theological note, I would also question the assertion of the hymn that "on earth is not [Satan's] equal." Christ remains present on the earth in the Blessed Sacrament and in his Church, and the Holy Spirit is surely with us.

From Wiki (The last paragraph strikes me as very interesting, as the wiki writer considers the inclusion of the hymn in a Catholic hymnal to be a Catholic endorsement of the hymn):

"A Mighty Fortress" is one of the best loved hymns of the Lutheran and Protestant traditions. It has been called the "Battle Hymn of the Reformation" for the effect it had in increasing the support for the Reformers' cause. John Julian records four theories of its origin:

Heinrich Heine: it was sung by Luther and his companions as they entered Worms on April 16, 1521 for the Diet;
K.F.T. Schneider: it was a tribute to Luther's friend Leonhard Kaiser, who was executed as a Protestant martyr on August 16, 1527;
Jean-Henri Merle d'Aubigné: it was sung by the German Lutheran princes as they entered Augsburg for the Diet in 1530 at which the Augsburg Confession was presented; and
the view that it was composed in connection with the Diet of Speyer (1529) at which the German Lutheran princes lodged their "protest" to Emperor Charles V, who wanted to enforce his Edict of Worms (1521).

Rare early printing of "A Mighty Fortress."The earliest extant hymnal in which it appears is that of Andrew Rauscher (1531), but it is supposed to have been in Joseph Klug's Wittenberg hymnal of 1529, of which no copy exists. Its title was Der xxxxvi. Psalm. Deus noster refugium et virtus.[1] Before that it is supposed to have appeared in the Hans Weiss Wittenberg hymnal of 1528, also lost.This evidence would support its being written in 1527-1529, since Luther's hymns were printed shortly after they were written.

Tradition states that King Gustavus Adolphus of Sweden had it played as his forces went to battle in the Thirty Years' War. The psalm had been translated into Swedish already in 1536, presumably by Olaus Petri. In the late 1800s the song also became an anthem of the early Swedish socialist movement.

It was first translated into English by Myles Coverdale in 1539 with the title, Oure God is a defence and towre. The first English translation in "common usage" was God is our Refuge in Distress, Our strong Defence in J.C. Jacobi's Psal. Ger., 1722, p. 83.

The hymn is now a suggested hymn for Catholic masses, appearing in the second edition of the Catholic Book of Worship, published by the Canadian Conference of Catholic Bishops. Its enduring popularity in Western Christendom has breached boundaries set in the Reformation.

The wiki article leans heavily on John Julian's Dictionary of Hymnology, which can be found here http://www.ccel.org/ccel/julian_j/hymn1.html
and here http://www.ccel.org/ccel/julian_j/hymn2.html