As Sure as the Dawn (Mark of the Lion #3) by Francine Rivers

Following A Voice in the Wind and An Echo in the Darkness, As Sure as the Dawn continues the chronicles of Hadassah, a Christian slave woman living during the height of the Roman Empire. She has saved the life of the scorned child of a disreputable Roman woman and the Germanic gladiator, Atretes. For her faith, Hadassah now languishes in condemnation, awaiting death in a dungeon beneath the arena.

Atretes, who holds fast to his dreams of revenge for the slaughter of his people, wants his son back. So he seeks out John the Baptist, who is the key to finding the custodian of his son, and brings his war-weary soul closer to redemption.