Remember, the Judaizers to which Paul was responding in Galatians were not crass perfectionists. They would have confessed that God’s grace was sufficient. They would have confessed the centrality of the atonement. They would have rejected explicit requirements of perfection. But they still insisted on just a few key teachings of the Law (distinctive boundary markers like circumcision, kosher eating, festivals, etc).
In other words, the harshness of Paul was NOT directly toward the Spencer Kimballs of the world. It was to the Robert Millets and Stephen Robinsons and Brad Wilcoxes of the world. Galatians is not directly written against a book like Miracle of Forgiveness, it is rather written against a book like Believing Christ. The compassionate rage of Paul in Galatians is against the subtlety of neo-orthodoxy.