The St. Christopher medal is a small metal medallion which Jonas Kahnwald found at the lake and later gave to Martha Nielsen. He is first seen with it at the beginning of Season 2, taking it with him after waking up in the derelict Kahnwald home.
In Christian tradition, Saint Christopher (Latin: Christophorus; German: Christoph) was a third century martyr. He is a patron saint of travelers, unmarried men, and sailors among many others, and is invoked for protection against storms, floods, and lightning. The church in Winden is dedicated to him.
According to legend, as a youth, he sought to follow the greatest king in the world. When the king crossed himself at the name of the devil, he decided the devil was stronger, and sought to follow the devil. But finding that the devil feared Christ, Christopher decided to follow Christ instead. His name is derived from Greek, meaning "Christ bearer," after another legend in which he ferries a small child across a raging river, who is then revealed to be Jesus.
Devout travelers may carry medallions of St. Christopher for good luck; like Jonas', they depict the saint carrying a child on his back.