Indeed, if gameness and heart are character traits that are valued highly in the prizefighter, so are they in the Christian.
"Jonathan Edwards just laid me bare in 1971 and '72 when I was reading his book Religious Affections. I can remember several nights where, in his chapter on evangelical humiliation, he began to peel back the onion layers of my soul. He would say, 'So you think you're humble? What if you're boasting in your humility?' And you admit, 'Yes, I probably am boasting in my humility.' And he would ask, 'Well, what if your confession that you are boasting in your humility is really a pose, and you're still boasting in your humility?'
He gave question after question that made you realize, 'There's no center to this onion.' You peel and peel and peel, and the last peel just disappears, because you can always ask yourself, 'How do you know?' You can always doubt yourself. There's no way, by mere self-analysis, to come to a point where you're looking at something that you can say, 'Definitely authentic!' Because the capacity of the human brain to doubt is always there.
So where in the world does assurance come from? The answer is that, even though introspection is commended and wise up to a point, the bottom line of assurance comes when you stop analyzing and you look to Christ and you look and you look and you look until Christ himself in his glory and his sufficiency by reflex, as it were, awakens a self-forgetful 'Yes!' to him.
Your best moments of assurance are not the moments when you're thinking about your assurance. Because the very moment that you're thinking about your assurance, you have the capacity at that moment to doubt your assurance. This little voice, whether it's your conscience or the devil, is saying, 'You think you have assurance, but...'
And so the answer comes, 'Look to the cross! Look to Christ!' And if you're able to look to the cross, if you're able to see him as sufficient and satisfying and powerfully able to carry all your sins, and you find yourself drawn out of yourself to say 'Yes' to him, that's what you want. You are assured. He is your assurance at that moment."
- John Piper, How can I know if my repentance is genuine?