In literary commonplace, Muslims do call Christians "infidels"; here the opposite occurs, and it is probably due to the fact that the story is set in the Holy Land where the two armies 'mirror' one another. But usually, during the Middle Ages and Renaissance, Christians used to refer to Muslims as gentiles (in Latin) or "pagani" (in Italian: pagans, heathens).
The latter term comes from the Latin word pagus, i.e. village, as traditional religions lasted for a much longer time in the European countryside after towns and cities had already converted to Christianity.
There even existed the Italian word "Paganìa" literally meaning 'heathenland' and indicating any non-Christian country, from North Africa and Arabia to India and China. As a matter of fact, in Tasso's Gerusalemme Conquistata all these peoples gather for the war, even though - of course - it was not so in the 'real' Crusade.