The GB was prepared by Cardinal Giulio Santorio to incriminate Cardinals Marcantonio and Ascanio Colonna. His motives were a combination of revenge, self-defense, ideological zeal and political leverage. The work itself - called a Gospel of Barnabas - is supposed to be the "narrative and miracles and doctrines" described as travelling with the apostle Barnabas in the 5th C apocryphal work Acts of Barnabas. The origins of the work are probably Cyprus and it enters Europe along with supporting literature through Milan probably shortly after 1571, its broader context being the fall of Cyprus to the Turks and the strategic shifts of Christendom in the counter-reformation following the Battle of Lepanto. As it survives, Santorio is using the work to incriminate the Colonna and expose a tradition of heresy and treachery among the nobility and high clergy.
http://www.latrobe.edu.au/arts/barnabas/marino.html