Life of Willibrord: Apostle to the Frisians
"A storm once drove a man of god to this island. He stayed until the storm lifted giving him an opportunity to sail away. He witnessed the island's idiotic religion. He also saw the island's cruel king condemn violators of the spring's sacrality to terrible deaths. He invoked the trinity and baptized three men in the spring. He also planned to kill some of the island's animals for food. The pagans looked on, think the Christians would swiftly die or go mad. But when they saw the violators suffer no harm, they fell into a terrified bewilderment and then told King Radbod what they had seen."
Driven mad because their gods did not lift a hand to smite their foe sounds like Christian propoganda.