That even the most hardened of people can change with kindness and compassion. The bishop demonstrated to Jean in a way he could understand that he was cared for, that he could Begin to trust others again, and there was nothing preventing him from changing and becoming an honorable man with self-respect.
The Bishop wanted to help him and the bishop understood that he was facing bigger challenges in his life and he had ever faced and so his compassion was genuine which of course made it even more believable to Jean and giving him the strength to believe it was chilly possible to follow the road of good and turn away from evil.