Great idea Maureen
Hmm - I didn't really find this part to be very significant and read it without much thought. Perhaps it's significance will become clearer later in the story. I'd be interested in other thoughts on this.
I suppose the bishop was surprised because he expected the revolutionary to be anti-authority, and hence anti-God. Actually he turned out to be someone who beleived in a greater good - justice, which demostrated that their hearts were much more closely aligned than he first thought.
He probably stole the silver because he had so hammered the idea of being 'owed' something by society into himself, and so desired revenge on all mankind, that he saw the silver as an opportunity for just such compensation and retribution.
Why did the mercy affect him so much? He had been treated incredibly unjustly and that resulted in an unswerving desire for revenge. Then When he himself treated someone unjustly, to be met with uncompromising mercy totally devoid of any such need for revenge, it was a complete revelation to him, and spoke deeply to his own heart and conscience.