Immediately after leaving the conclave [8 April 1455], our soul oppressed by the fall and misfortune of Constaninople [1453] we made a vow to embark on a war against the Turks, to those who are iniquitous usurpers of Christian lands. Therefore, given that our funds are not enough, we decided to gather the armies of all the pirnces of the arth who profess the religion of Jesus Crucified-- you among them.... Between Westerners and Easterners, we have organized two good armies of land and sea, of sufficient valor.... we plan to add to the enterprise your important cooperation, with which we will have enough strength no only to defeat the unholy Turk, but to take away from the infidel the holy land of Jerusalem.... Because God wanted you to have, under your sublime rule, a powerful army, the river Nile whose inundations fertilize the land where our enemies feed and you at your own pleasure can take that away from them.
Salvadore (p.66) adds that this letter is particularly notable for being the first explicit "call to arms" sent to the Ethiopian ruler, the presumed Prester John.