Jesus has been unloading a bunch of spiritual Truth on the disciples becasue He is getting ready to face His death. This is natural in fact, in verse 4 Jesus tells them He didn't say anything earlier becasue there was time... now the time is up and He's got to get them prepared.
After unlaoding all this stuff the disciples finally say, "...from this we believe that you came from God." But Jesus knows better and responds, "...do you finally believe? But the time is coming when you will be scattered and each of you will leave me alone."
Here's some quick thoughts on belief and faith - don't hang me by them, just quick thoughts.
You can believe in God all day but what do you do when when its dark and scary?
Faith takes over when what your belief in something runs out.
Belief is mental - faith has feet.
It's easy to believe when there is no pressure. Pressure reveals the faith belief talks about.
Faith happens where your belief and the unknown make contact.
If you believe in God but never have to test it through trial there is no faith. Belief comes natural, faith is developed in the fire.
That should get you thinking!