God sees nothing wrong in turning sinners into believers and then use them in a mighty way to to fulfill His purpose. Moses and Paul are two good examples.
The problem is that human beings find it hard not only to accept such people but to believe that God can use them.
Even when they find themselves in a situation where they need divine interference, they expect God to send angels. If God sends someone with a pathetic history that they know about they refuse to accept them. They find it hard to believe that God can use such people.