Google gives me two definitions for "crusade"

The first refers to the actual Crusades. The second refers to a general quest.

Is Indiana Jones and the Last Crusade meant to refer to either one of these definitions?

As far as I know, there are no Muslims in the film and at no point do Muslim and Christian characters fight, so the first doesn't really fit.

Indy does arguably go on a crusade, to get The Holy Grail, but there is nothing to indicate it will be his last.

So why is the film called The Last Crusade?