Thomas Cajetan, OP, also known as Gaetanus, commonly Tommaso de Vio or Thomas de Vio, was an Italian philosopher, theologian, the Master of the Order of Preachers 1508 to 1518, and cardinal from 1517 until his death. He was a leading theologian of his day who is now best known as the spokesman for Catholic opposition to the teachings of Martin Luther and the Protestant Reformation while he was the Pope's legate in Augsburg, and among Catholics for his extensive commentary on the Summa Theologica of Thomas Aquinas.

Martin Luther was a German priest, theologian, author, hymnwriter, professor, and Augustinian friar. Luther was the seminal figure of the Protestant Reformation, and his theological beliefs form the basis of Lutheranism. He is regarded as one of the most influential figures in Western and Christian history.