Thought by many to be a convert from Judaism, Alonso de Espina was for many years superior of the House of Studies of the Friars Minor at Salamanca, Spain.
[2] He was a man of great learning and attained considerable renown as a preacher, but his chief claim to fame was being the author of Fortalitium Fidei.
The edition published at Nuremberg in 1485 begins thus: Incipit prohemium Fortalitii Fidei conscriptum per quendam Doctorem eximium ordinis minorum anno MCCCCLIX in partibus occidentis (Here begins the introduction to "Fortalitium Fidei" (Fortress of Faith) written by a certain distinguished teacher of the Order of Friars Minor in the year 1459).The fact that the Fortalitium appeared anonymously gave rise to some difference of opinion as to its authorship.
It is divided into five[3] books: the first directed against those who deny the divinity of Jesus; the second against "heretics"; the third against the Jews; the fourth, against Muslims; the fifth gives instructions on the battle against the devil.
Alonso de Espina published at least three other works: In The Complete Book of Devils and Demons[4] Leonard R. N. Ashley says that Alonso is quoted as stating that the total number of angels who sided with Lucifer's revolt was 133,306,668,[5] a figure, Ashley remarks, so precise that one hardly knows what to say; he adds that the Book of Enoch puts the number at 200.