After setting forth the rule that sums up the Law and the Prophets, the Lord now warns how difficult it is to live according to it. For though the command is simple, the way of obedience is not easy. The narrow gate signifies the discipline by which the soul is restrained from excess, pride, and self-indulgence, while the broad way represents the liberty of following one’s own desires without correction.

Many choose the broad way, not because they are ignorant of the good, but because they prefer ease to truth. For the broad way offers immediate pleasure and freedom from restraint, yet it leads to ruin because it leaves the heart unformed and the will ungoverned. The multitude follows this path because it requires no effort to deny oneself.

The narrow way, by contrast, demands perseverance, humility, and patience. It is narrow not because it is unjust, but because it excludes what is harmful. It is strait because it allows no wandering into excess or negligence. Those who walk this way submit their desires to discipline, and by this very restraint they are led to life.

That few find it does not make the way uncertain, but precious. For truth is not made false by being rare, nor is goodness diminished because it is difficult. Rather, the small number of those who persevere shows how great is the reward promised, since it is not obtained without effort.

Thus the Lord exhorts His hearers to choose deliberately, not to follow the crowd, but to pursue life through self-denial. The narrowness of the path is borne only for a time; the life to which it leads has no end. In this way the soul is taught to endure present hardship for the sake of eternal joy, preferring the strait way that heals to the broad way that destroys.

Source: St. Augustine, On the Lord’s Sermon on the Mount,  Book II, Chapters 21-22