The Road to Jerusalem by Jan Guillou

Arn Magnusson, born into an aristatic Swedish family, is raised in an old monastery because of an old promise made by his mother. From the start he shows the natural skill and aptitude of a born fighter, yet despite his strength he is innocent in the ways of the world. He is sent from the sheltered walls of the convent to experience something of real life. On his journeys Arn falls foul of various fighting groups, but is also delighted with the women he encounters. Seduced by one sister, he falls in love with the other and ends up sleeping with them both - a mortal sin in the medieval church. While his love is sent to a convent, Arn's sentence is commuted to forced commitment to the cause of the Crusades, where he becomes a notable soldier and eventually a high ranking commander of the Knights Templar and both friend and enemy to the charismatic Saladin.
Arn's Story continues in the next two books: The Knight Templar and Kingdom at the End of the Road.

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