Henry started to write it in 1518, while he was reading Martin Luther's attack on indulgences. By June of that year, he had shown it to Thomas Wolsey, but it remained private until three years later, when the earlier manuscript became the first two chapters of the Assertio, the rest consisting of new material relating to Luther's De Captivitate Babylonica. It is believed that Thomas More was involved in the composition of the piece.
Scarisbrick describes the work as "one of the most successful pieces of Catholic polemics produced by the first generation of anti-Protestant writers." It went through some twenty editions in the sixteenth century, and as early as 1522 had appeared in two different German translations.
It was dedicated to Pope Leo X, who rewarded Henry with the title Fidei Defensor (Defender of the Faith - a title still used by the monarchs of the United Kingdom) in October 1521. This takes on a certain irony, when one considers the religious upheavals that would shake England in the Protestant Reformation of the ensuing decades. Successive English and British monarchs have retained this title, even after the Anglican Church broke away from Catholicism.