Some very important thoughts on indiscreet zeal...

"If exactitude be praiseworthy, severity is blamable."
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"Remember that God, to punish those who have practised [this] indiscreet zeal, and to correct them, has often allowed them to fall into much graver faults than those which has scandalised them in others.
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[Therefore] I command you never to speak of God, or of anything good, unless in a spirit of humility and meekness, in an amiable and gracious manner, with moderation and encouragement, and never with bitterness and severity, or in a way to wound and repel those who hear you, because, although you may only say what is in the Gospel and in the best books, I believe that in your present state of mind you might say it very badly and in such a way as only to do harm. Did not Satan make use of the words of Holy Scripture to tempt our Lord?
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Truth is the proper relation of things. It is changed when pushed to extremes, or wrongly applied. Your peevish temper is like a smoked glass, which, if you do not take care will prevent you seeing things in their true light, or showing them to others. Keep always on your guard against this fatal influence, and feed your mind on thoughts and feelings that are contrary to those inspired by temper. Entertain yourself and others with conversations on the infinite goodness of God, and on the confidence we ought to have in Him.
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Compel yourself to offer an example in your whole conduct, of a virtue that has no bounds, and which imposes no restraint on others. If you have nothing kind to say keep silent, and leave the care of deciding to others. They can avoid better than you too much laxness, and will be exact without being severe. If exactitude be praiseworthy, severity is blamable, it does nothing but revolt people instead of convincing them, and embitter their souls instead of gaining them. As much as true meekness, with the help of God, has power to repel evil and to win to good, so much has an excessive harshness power to make goodness difficult and evil incurable. The first is edifying, the latter, destructive." - Letter XI, Intemperate Zeal, De Cassaude, Letters