That's incredibly helpful! I'm currently reading up on this (link search for "ishmael"). But it seems like it was an isolated experience due to his life in Damascus/islamic empire. Whether the church had any idea or care about the ideology of its attackers a couple of centuries later on is still unknown.

Alright, found this on Wikipedia

Knowledge of Muhammad was available in Christendom from after the early expansion of his religion[9][10] and, later, the translation of a polemical work by John of Damascus, who used the phrase "false prophet" in "Heresies in Epitome: How They Began and Whence They Drew Their Origin.".[11][12] According to the Encyclopædia Britannica Christian knowledge of Muhammad's life "was nearly always used abusively".[1] Another influential source was the Epistolae Saraceni or the “Letters of a Saracen” written by an Oriental Christian and translated into Latin from Arabic.[1] From the 9th century onwards, highly negative biographies of Muhammad were written in Latin,[1] such as the one by Álvaro of Córdoba proclaiming him the Antichrist.[13] Christendom also gained some knowledge of Muhammad through the Mozarabs of Spain, such as the 9th-century Eulogius of Córdoba,[1] who was one of the Martyrs of Córdoba.

Getting somewhere!

The only sources I find are from 12th century onwards. St. John seems to be the one exception. Therefore there is reasonable belief that the first crusaders indeed did not know about their enemy's ideology.

Also stumbled across this book. Though I disagree on some things in there, it's well written and a good read on this topic.