This is Martin Luther’s rule for dealing with false teachers and the falsely taught. The false teachers are dealt with severity while the falsely taught are dealt with compassion.
Here’s Luther on Galatians 1:5 (Greater Galatians Commentary, 1535):
“Here Paul lives up to his own rule, which he gives later on in the sixth chapter (Galatians 6:1): “Brethren, if a man is overtaken in any trespass, you who are spiritual, etc.” We, too, should follow this example. We should show that toward those poor disciples who have been led astray we feel as parents feel toward their children, so that they may see our paternal zeal and maternal feelings toward them and may see that we seek their salvation. But when it comes to sectarianism, we should follow the example of the apostles. We should be impatient, proud, sharp, and bitter, despising and condemning their sham as sharply and harshly as we can. When a child has been bitten by a dog, the parents chase the dog but console and soothe the weeping child with the sweetest of words.”