By St. Isaac the Syrian
A zealous man never achieves peace of mind. But he who is a stranger to peace is a stranger to joy. If, as it is said, peace of mind is perfect health, and zeal is opposed to peace, then the man who has a wrong zeal is ill with a grievous disease. Though you presume, a man, to send forth your zeal against the infirmities of other men, you have expelled the health of your own soul; be assiduous, rather, in labouring for your own soul’s health.
If you wish to heal the infirm, know that the sick are in greater need of loving care than of rebuke. Therefore, although you do not help others, you expend labour to bring grievous illness upon yourself.
Zeal is not reckoned among men to be a form of wisdom, but as one of the illnesses of the soul, namely narrow-mindedness and deep ignorance.
The beginning of divine wisdom is clemency and gentleness, which arise from greatness of soul and the bearing of the infirmities of men. For, he says, ‘Let the strong bear the infirmities of the weak’, and ‘Restore him that has fallen in the spirit of meekness.’ The Apostle numbers peace and patience among the fruits of the Spirit.
From Homily 51.