Byzantium by Stephen Lawhead. The story begins at an isolated Irish monastery in the ninth century. There we meet a young and idealistic monk named Aidan. His life follows a simple routine, but he is more than fulfilled in his duties as a scribe, charged with helping to prepare a magnificent manuscript for the Holy Roman Emperor. When it is finished, Aidan is selected to join the quest to deliver this treasure to the far-off city of Constantinople, the royal seat of the kingdom of Byzantium (the new Rome).
As the small group of monks head out on their trek, it is not long before they run into danger. A group of Danish barbarians known as the Sea Wolves attacks the small, defenseless crew, and in their treasure lust strike down a number of the monks. Before Aidan can learn what has happened to his brothers, he is taken by the Danes as a slave and brought back to their lands. Yet, he learns to accept his role and serves his new masters well, earning their respect and trust. As their small village raids are deemed insufficient, the Danes are soon called into council by their king to travel to a far off land where it is rumored the roads are paved with gold and silver. An opportunity the Sea Wolves cannot pass up. Thus Aidan and his slave masters set off on their own quest to Miklagard, where even the lowliest slaves are more wealthy than the Danish lords.
Miklagard, to Aidan's surprise, turns out to be Byzantium and, upon his arrival, he is determined to find out if any of his brother monks from Ireland survived the barbarian raid and completed their quest. The Danes are also in for a surprise as they find the city to be set out on a scale that their tiny imaginations just cannot fathom. They are forced to change their plans and swear fealty to the emperor in order to claim the treasure that they seek. Aidan gains favor with the Byzantines due to his demeanor and ability to translate between Danish, Latin, and Greek. It is in the city of gold that his life changes again when he is enlisted by the emperor for a special task that appears to cost him everything, his life, his hope, and his faith. Yet despite his human failings and his imminent execution, Aidan has a destiny to fulfill. It seems his God is not yet through with him as he had come to believe.