The Day of Saladin

(from ebookjapan):

At the end of the 12th century, Saladin, an anti-crusader hero emerged from Islam. The Knights are burning with the will to fight in order to take back the holy city of Jerusalem from Saladin. In the midst of all this, Hugues of the Knights Templar, Paolo of the Knights of John, and Nicolas of the Kingdom of Jerusalem are sent to Gaza with just three men.

The journey of these monastic knights, who have renounced the secular world and vowed poverty and chastity, to become the three true heroes begins...! Yasuko Aoike's masterpiece of medieval adventure romance!