This unusual and surprising illustration is a picture of the inappropriateness of honouring the fool. The sling is used to hurl a stone at great speed at a target.
Note that God will never give honour to those who are not worthy of it. He never flatters man, or looks at him unrealistically, or misreads is character. Though God justifies the wicked, he knows that they are wicked and is not in any way deceived about them. When he justifies, he at the same time begins the work of sanctification in their hearts and causes them to die to sin. God does not call those his children who are not his children, for the effect of this would be to encourage men and women to be deluded about their state.
We may apply this to those who preach a watered-down gospel without any mention of sin. They tell of all the so-called benefits of the Christian life, but they will not preach for conviction. They give too much honour to the fool. The result is that no one is convicted or converted. They ruin the church by bringing in wood hay and stubble, and they pervert the gospel by silencing God and making the gospel something weak and powerless.