The question, then, that the example of Judas and Peter boils down to is not whether you are, in your heart, a good person; we should argue that both of these men were good. The question is whether or not you are teachable, particularly in points where it comes to your faith.
Most of us aren't, in my experience, as teachable as we'd like to think that we are. We're mostly interested in acquiring new information that we can fit into our existing frameworks, new ideas that go smoothly into old paradigms. It's called a confirmation bias - we seek out that which confirms what we're sure that we already know. We call this learning, and we call ourselves teachable, but the truth is that we never truly learn anything at all, except that "we were right all along."
It makes us arrogant, but that's not even the biggest problem with it. The real problem is that it keeps us from being actually knowledgeable in any real way, and that means that the truth that we're so sure we're attached to still eludes us.
Judas was not teachable. He had an idea of what this whole Jesus thing was about, and that's what he went after. He pursued it with everything that he had, and when the whole thing turned out to be something different, there was no room in his heart to expand his definition and embrace it. The truth broke him, and he ended up dead in a field. It was really the only possible end for him once his knowledge had been stretched to its breaking point, as he was not a teachable man and could not make room for any different knowledge.
As Jesus Himself said, you can't put new wine in old wineskins; the skins break and the wine runs out and both are ruined. That's what happens when you're not teachable.
On the other hand, Peter continually learned from his experiences. He watched Jesus intently and listened to Him, and although we see Peter as impetuous and as quick to speak without real understanding, we also see him wrestling - often, out loud - with new ideas. He wants this whole Jesus thing to be what it is, and he will bring himself to understand it as best he can. His knowledge keeps expanding, bringing in new ideas along with old ones and attempting to make a synthesis out of them. He's not a perfect learner - he doesn't always get it right - but he is teachable. He's trying. And when it breaks, he's grieved, but not defeated.
That's the difference.
Judas wanted to be a part of the whole Jesus thing; Peter wanted the whole Jesus thing to be a part of him. Judas wanted to help form it, based on all that he knew about what it was; Peter wanted it to form him, knowing it was beyond what he already understood.
Which brings the question back to us - are you teachable?
When was the last time you learned something you didn't already know? Something radically different from what you thought you knew? When was the last time you let a new truth shape you, rather than trying to shape it to fit inside a mold that you already have?
When was the last time you willfully purchased a new wineskin, knowing that what you had was insufficient for the new wine of faith being poured into it?
The truth is that for most of us, the answer is either so far in the past that we have forgotten precisely when or it is never at all, for we just assume that we already know everything we need to know about God for the sake of our faith. We already have our ideas, our foundations, and anything else is just bonus on top of that. Anything else just makes deeper the faith holes we've dug for ourselves.
But what if that wasn't the case? What if we could learn things to make our faith broader, not deeper? What if we could embrace a bigger vision of God, not just a more intense one? What if the mystery of God truly is bigger than our ability to fathom it and we have to keep getting new wineskins, just so that we don't burst with it all?
I think it is. I mean, I know that it is. Everything my life has taught me about the faith screams that it is. Which leaves but one question to be answered:
Are you teachable?