It happens when someone close to him is threatened. We learned that he's capable of fighting, but in such a poor and clumsy way that leaves one wondering if he's truly serious about protecting anything at all.

[She answers coolly, severely, very much showing a side of her that her allies are seldom privy to. That 'priest' is a useless good-for-nothing, it's Ciel's objective and unbiased conclusion over his character, that's all.]