Many times I have not known myself. When I was in my twenties and divorced, my ex-husband had a girlfriend and lived with her. My daughters would go visit and I hated the fact my ex wasn't married and living in open sin. I thought, "I would never have someone come sleep with me with my daughters in the house!" Two years later, I did just that.
After a few incidents like that, I was finally humbled to realize I was capable of any sin, no matter what I thought. One day at church we were singing, "I Will Not be Shaken," and I turned to my mother and said, "On the other hand, who knows WHAT I will do?"
Jesus told Peter, "You will deny me." Peter was horrified and said he would never do that. He said he was willing to die with him. The other disciples said the same. We know how that turned out.
I wondered if God sometimes lets us fall flat on our face so we can see how weak and sinful we are. It humbles us and we realize how we must have Jesus do everything for us. He can keep us from falling, if we acknowledge we have no strength to do it ourselves.
I came across the story of Hezkekiah the other day. It says that God left Hezekiah to show him what was truly in his heart. Verse 26 says his heart was full of pride.
"And so in the matter of the envoys of the princes of Babylon, who had been sent to him to inquire about the sign that had been done in the land, God left him to himself, in order to test him and to know all that was in his heart." 2 Chronicles 32:31
Matthew Poole's Commentary
Matthew Henry's Concise Commentary.
God left Hezekiah to himself, that, by this trial and his weakness in it, what was in his heart might be known; that he was not so perfect in grace as he thought he was. It is good for us to know ourselves, and our own weakness and sinfulness, that we may not be conceited, or self-confident, but may always live in dependence upon Divine grace. We know not the corruption of our own hearts, nor what we shall do if God leaves us to ourselves. His sin was, that his heart was lifted up. What need have great men, and good men, and useful men, to study their own infirmities and follies, and their obligations to free grace, that they may never think highly of themselves; but beg earnestly of God, that he will always keep them humble! Hezekiah made a bad return to God for his favours, by making even those favours the food and fuel of his pride.