A monk must be extremely cautious of carnal and animal zeal, which outwardly appears pious but in reality is foolish and harmful to the soul.
Worldly people and many living the monastic life, through ignorance and inexperience, often praise such zeal without understanding that it springs from conceit and pride. They extol this zeal as zeal for the faith, for piety, for the Church, for God. It consists in a more or less harsh condemnation and criticism of one’s neighbours in their moral faults, and in faults against good order in church and in the performance of the church services. Deceived by a wrong conception of zeal, these imprudent zealots think that by yielding themselves to it they are imitating the holy fathers and holy martyrs, forgetting that they – the zealots – are not saints, but sinners.
When for your labour in the garden of the commandments God grants you to feel in your soul divine zeal, then you will see clearly that this zeal will urge you to be silent and humble in the presence of your neighbours, to love them, to show them kindness and compassion, as Saint Isaac the Syrian has said.
Divine zeal is a fire, but it does not heat the blood. It cools it and reduces it to a calm state. The zeal of the carnal mind is always accompanied by heating of the blood, and by an invasion of swarms of thoughts and fancies. The consequences of blind and ignorant zeal, if our neighbour opposes it, are usually displeasure with him, resentment, or vengeance in various forms; while if he submits, our heart is filled with vainglorious self-satisfaction, excitement and an increase of our pride and presumption.
St. Ignatius Brianchaninov