After warning of the narrow way, the Lord now cautions against guides who would mislead those striving to enter it. For many appear gentle and harmless outwardly, yet inwardly are driven by pride, ambition, or deceit. Such men do not openly oppose the truth, but corrupt it under the appearance of righteousness.
The clothing of sheep signifies the external marks of piety—words, gestures, or offices that inspire trust. The wolf is hidden beneath when the heart is ruled by desire for power, praise, or gain. The danger lies not in open hostility, but in concealed corruption, which deceives by resemblance to what is good.
The Lord therefore gives a simple and reliable test: fruits, not appearances. Teaching is known by the life it produces. Just as the nature of a tree determines the kind of fruit it bears, so the interior disposition of a man shapes his actions. Good fruit proceeds from charity; evil fruit from pride and self-seeking.
This does not mean that a single act reveals the whole man, but that a consistent pattern of life manifests the root from which it springs. Words may be fashioned to deceive, but deeds disclose the truth. What one truly loves is eventually made visible in how one lives.
The saying that a good tree cannot bear evil fruit does not deny human weakness, but teaches that when the root is healed, the fruit is ordered toward good. Conversely, where the heart remains corrupt, even actions that seem good are spoiled by a disordered intention. Thus the Lord judges not the outward form, but the inward source.
The final warning recalls the seriousness of this discernment. Those who do not bring forth good fruit are cut down, not because they lacked outward success, but because they lacked true righteousness. Therefore the disciple is taught to be watchful, neither trusting appearances nor following blindly, but examining fruits with humility and care, lest the way that leads to life be obscured by false guides.
Source: St. Augustine, On the Lord’s Sermon on the Mount, Book II, Chapters 23-24