For indeed holy teachers take unto themselves a tile, when they lay hold of the earthly heart of hearers in order to teach them: which tile in truth they lay before themselves, because they keep watch over it with the entire bent of their mind: on which tile also they are commanded to pourtray the city Jerusalem, because they are at the utmost pains to represent to earthy hearts by preaching a vision of supernal peace. But, because the glory of the heavenly country is perceived in vain, unless it be known also what great temptations of the crafty enemy assail us here, it is fitly subjoined, And you shall lay siege against it, and build forts. For indeed holy preachers lay siege about the tile on which the city Jerusalem is delineated, when to a mind that is earthy but already seeking after the supernal country they show how great an opposition of vices in the time of this life is arrayed against it. For, when it is shown how each several sin besets us in our onward course, it is as though a siege were laid round the city Jerusalem by the voice of the preacher. But, because preachers ought not only to make known how vices assail us, but also how well-guarded virtues strengthen us, it is rightly subjoined, And you shall build forts. For indeed the holy preacher builds forts, when he shows what virtues resist what vices. And because, as virtue increases, the wars of temptation are for the most part augmented, it is rightly further added, And you shall cast a mount, and set camps against it, and set battering rams round about. For, when any preacher sets forth the mass of increasing temptation, he casts a mount. And he sets camps against Jerusalem when to the right intention of his hearers he foretells the unsurveyed, and as it were incomprehensible, ambuscades of the cunning enemy. And he sets battering-rams round about, when he makes known the darts of temptation encompassing us on every side in this life, and piercing through our wall of virtues.
But although the ruler may nicely insinuate all these things, he procures not for himself lasting absolution, unless he glow with a spirit of jealousy against the delinquencies of all and each. Whence in that place it is further rightly subjoined, And do thou take to you an iron frying-pan, and you shall set it for a wall of iron between you and the city. For by the frying-pan is denoted a frying of the mind, and by iron the hardness of reproof.
But what more fiercely fries and excruciates the teacher's mind than zeal for God? Hence Paul was being burnt with the frying of this frying-pan when he said, Who is made weak, and I am not made weak? Who is offended, and I burn not?. And, because whosoever is inflamed with zeal for God is protected by a guard continually, lest he should deserve to be condemned for negligence, it is rightly said, You shall set it for a wall of iron between you and the city. For an iron frying-pan is set for a wall of iron between the prophet and the city, because, when rulers already exhibit strong zeal, they keep the same zeal as a strong defence afterwards between themselves and their hearers, lest they should be destitute then of the power to punish from having been previously remiss in reproving.
But meanwhile it is to be borne in mind that, while the mind of the teacher exasperates itself for rebuke, it is very difficult for him to avoid breaking out into saying something that he ought not to say. And for the most part it happens that, when the faults of subordinates are reprehended with severe invective, the tongue of the master is betrayed into excess of language. And, when rebuke is immoderately hot, the hearts of the delinquents are depressed to despair. Wherefore it is necessary for the exasperated ruler, when he considers that he has wounded more than he should have done the feelings of his subordinates, to have recourse in his own mind to penitence, so as by lamentations to obtain pardon in the sight of the Truth; and even for this cause, that it is through the ardour of his zeal for it that he sins. This is what the Lord in a figure enjoins through Moses, saying, If a man go in simplicity of heart with his friend into the wood to hew wood, and the wood of the axe fly from his hand, and the iron slip from the handle and smite his friend and slay him, he shall flee unto one of the aforesaid cities and live; lest haply the next of kin to him whose blood has been shed, while his heart is hot, pursue him, and overtake him, and smite him mortally. For indeed we go with a friend into the wood as often as we betake ourselves to look into the delinquencies of subordinates. And we hew wood in simplicity of heart, when with pious intention we cut off the vices of delinquents. But the axe flies from the hand, when rebuke is drawn on to asperity more than need requires. And the iron leaps from the handle, when out of reproof issues speech too hard. And he smites and slays his friend, because overstrained contumely cuts him off from the spirit of love. For the mind of one who is reproved suddenly breaks out into hatred, if immoderate reproof charges it beyond its due. But he who smites wood incautiously and destroys his neighbour must needs fly to three cities, that in one of them he may live protected; since if, betaking himself to the laments of penitence, he is hidden under hope and charity in sacramental unity, he is not held guilty of the perpetrated homicide. And him the next of kin to the slain man does not kill, even when he finds him; because, when the strict judge comes, who has joined himself to us by sharing in our nature, without doubt He requires not the penalty of his fault from him whom faith, hope and charity hide under the shelter of his pardon.
Source: Pastoral Rule (New Advent)