"Formerly he was useless to you, but now he has become useful both to you and to me" (Philem 11).
Paul's letter to Philemon reveals something about a man named Onesimus. At one time, Paul viewed Onesimus as useless. But while Paul was in chains he treated Onesimus as a son. Something changed in this man that made him useful instead of useless. When Jesus met Peter, he saw an impetuous man who drew quick conclusions and was very opinionated. I'm sure Jesus had his doubts about him for future leadership. However, Jesus saw something in Peter that was going to be useful once the rough edges were removed. Both of these men were simple fools in the Kingdom of God. The reference to someone being a fool was not necessarily a negative term. A simple fool, or peti, was a person who made mistakes, but quickly righted them and was restored to fellowship with God and with others. King David was a simple fool, one who made mistakes, but kept a repentant heart toward God. This is why God did not turn away from him for his many sins. The hardened fool, kesil and ewil, makes mistakes but never learns from them, is not repentant and will not listen to others. Such people can expect God's reproof to continue and they will eat the fruit of their own way (see Prov. 1:31-32). The hardened fool "returns to his own vomit." King Saul was a hardened fool, one who made mistakes and continued to do so even after realizing he was wrong. We are going to err in our ways. The question is, once we know we have made a mistake before God, do we make the necessary adjustments that will allow Him to intervene on our behalf? And will we avoid the same course of action in the future? God says that if we do, He will pour out His Spirit on us (see Prov. 1:23). When you work with people who have strong personalities but may be immature in their faith, you must discern if they are simple fools or hardened fools. This will tell you whether to invest time and resources into them.