Stanzas 1-3 of the original text were inspired by Psalm 46; stanza 4 arose directly from Luther's persecution experience. The text expresses trust in God's protection amidst the battle that Christians wage against the devil. "Earthly powers" in stanza four undoubtedly referred to the Roman Catholic authorities of Luther's day, but modern Christians may identity other "powers" that oppose the rule of Christ. The closing line of the text provides much comfort: "God's truth abideth still; his kingdom is forever!"
Bert Polman, Psalter Hymnal Handbook
The Christology of the song, “A Mighty Fortress Is Our God” is carefully based on the content of the Apostles’ Creed, and the firmness of the convictions here echo the words of Belgic Confession, Article 20, that God gave “his Son to die, by a most perfect love, and [raised] him to life for our justification, in order that by him we might have immortality and eternal life.” “When [his] benefits are made ours, they are more than enough to absolve us of our sins…(Additionally,) the Holy Spirit kindles in our hearts a true faith that embraces Jesus Christ, with all his merits, and makes him its own, and no longer looks for anything apart from him” (Belgic Confession, Article 22).