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, though 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 neighbors 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.
If the saints accused or convicted those who were living in sin or irreligion, they did so at the command of God, as their duty, by inspiration of the Holy Spirit, not at the instigation of their passions and demons. Whoever decides of his own self-will to convict his brother or make some reprimand, clearly betrays and proves that he considers himself more prudent and virtuous than the person he blames, and that he is acting at the instigation of passion and deception and diabolic thoughts. We need to remember the Savior's injunction: 'Why do you look at the speck in your brother's eye, but not notice the log in your own eye? Or how can you say to your brother, "Let me take the speck out of your eye," when there is a log in your own eye? You hypocrite, first take the log out of your own eye, and then you will see clearly to take the speck out of your brother's eye.' (Matt. 7:3-5)
What is a log in this connection? It is the earthly wisdom or carnal outlook, hard as a log, which deprives the heart and mind of all capacity for true vision, so that one is quite unable to judge either one's own inner state or the state of one's neighbor. such a person judges himself and others as he imagines himself to be, and as his neighbors appear to him outwardly, by his carnal mind (Rom. 8:6), mistakenly. And so the Word of God is extremely just in calling him a hypocrite.
A Christian, after being healed by the Word of God and the Spirit of God, gains a true view of his spiritual state and of that of his neighbors. the carnal mind, by striking his neighbor with a log, always upsets and confuses him, often ruins him, never does any good and cannot bring any benefit, and has not the least effect on sin. On the other hand, the spiritual mind acts exclusively on the soul-sickiness of one's neighbor, compassionates, heals and saves him...
If you want to be a true, zealous son of the Orthodox Church, you can do so by the fulfilment of the commandments of the Gospel in regard to your neighbor. Do not dare to convict him. Do not dare to teach him. do not dare to condemn or reproach him. To correct your neighbor in this way is not an act of faith, but of foolish zeal, self-opinion and pride. Poemen the Great was asked, 'What is faith?' The great man replied that faith consists in remaining in humility and showing mercy; that is to say, in humbling onseself before one's neighbors and forgiven them all discourtesies and offenses, all their sins. As foolish zealots make out that faith is the prime cause of their zeal, let them know that truth faith, and consequently also true zeal, must express themselves in humility regarding our neighbors and in mercy towards them. Let us leave the work of judging and convicting people to those persons on whose shoulders it is laid the duty of judging and ruling brethren.
--St. Ignatius Brianchaninov (d. 1867), The Arena, pp. 140-142