Theophylact of Ohrid

"How narrow is the gate and how hard the way which leadeth unto life, and few there be that find it!" The word "how" expresses the Lord's wonderment, as if He were saying, "Alas, how narrow it is!" But how is it that the Lord says on another occasion, "My yoke is light" (Mt. 11:30)? It is light on account of the future rewards.

Commentary on Matthew ↗

Gregory the Dialogist

Therefore he who eats the Passover ought to have his loins girded, so that he who celebrates the solemnity of resurrection and incorruption may no longer be subject to corruption through any vices, may subdue pleasures, and may restrain the flesh from lust. For he does not know what the solemnity of incorruption is who still lies subject to corruption through incontinence. These things are hard for some, but narrow is the gate that leads to life.

Forty Gospel Homilies, Homily 22 ↗

Thomas Aquinas

For the gate is narrow and the way is hard, that leads to life. This is contrary to the preceding. It is narrow, because it is restricted by the rule of the Law, and it is a way contrary to the other way: "For the ways on your right the Lord knows, but the ones on your left are perverse" (Pr 4:27). But one might ask why the way of charity is narrow, for it seems to be wide: "I have led you in the paths of uprightness. When you walk, your step will not be hampered: and if you run, you will not stumble" (Pr 4:11). …

Commentary on Matthew ↗