True zeal is connected with a holy life. It is remarkable how often the greatest zealots for God, the Church, and sound doctrine (as they regard it), have been unholy and even immoral in their lives.
The Galatians are severely censured for giving heed to false doctrines, and are called to pronounce even an apostle anathema, if he preached another gospel.
No more soul-destroying doctrine could well be devised than the doctrine that sinners can regenerate themselves, and repent and believe just when they please.