From Commentary on Romans (1515).

By “mystery” in verse 25 the apostle Paul means the mystery of Christ’s incarnation and redemption.  The Gospel is nothing else than the proclamation of Christ.  So Paul writes at the beginning of the same letter, in 1:1-4, “Set apart unto the gospel of God… concerning his Son Jesus Christ our Lord, who was of born of the seed of David and declared to be the Son of God with power, according to the spirit of holiness, by the resurrection from the dead.” 

Martin Luther (1483-1546) was a German priest and theologian, a seminal figure of the Protestant Reformation. His teaching about justification by faith, revealed in his study of the Pauline Epistles, became the core of Protestant teaching about salvation, and inspired a wide-reaching series of reform in Christian ministry, worship, and spiritual practice. He lectured on the Holy Scriptures for decades at the University of Wittenberg and wrote many Biblical commentaries. He is commemorated on February 18 on the calendars of several Lutheran and Anglican churches.