When Jesus was about to speak of the entrance of the narrow gate, He said first, strive, for unless the mind struggles manfully, the wave of the world is not overcome, by which the soul is ever thrown back again into the deep. For as in earthly life the departure from what is right is exceeding broad, so he who leaves the path which leads to the kingdom of heaven, finds himself in a vast extent of error. But the right way is narrow, the slightest turning aside being full of danger, whether to the right or to the left, as on a bridge, where he who slips on either side is thrown into the river. For the soul wavers to and fro, at one time choosing virtue when it considers eternity, at another preferring pleasures when it looks to the present.
The master of the house is Christ, who since as very God He is every where, is already said to be within those whom He gladdens with His visible presence though He is in heaven, but invisible to those whom He helps in secret while they are still contending in this earthly pilgrimage. But He will enter in when He shall bring the whole Church to the contemplation of Himself. He will shut the door when He shall take away from the reprobate all room for repentance. Who standing outside will knock, that is separated from the righteous, will in vain implore that mercy which they have despised.
These things apply to Christians. For we eat the body of Christ and drink His blood as often as we approach the mystic table at Mass, and He teaches in the streets of our souls, which are open to receive Him. The banquet does not delight one who is impious. The knowledge of the Scriptures does not make him known to God, whose iniquitous works proves to be unworthy; as it follows, "And he will say to you, I know not whence you are; depart from me."
Many at first burning with zeal, afterwards grow cold; many at first cold, on a sudden become warm; many despised in this world, will be glorified in the world to come; others renowned among men, will in the end be condemned.
On the Gospel of Luke (St. Bede the Venerable)
Explanations on the New Testament (St. Theophylact of Ochrid)
In Regulas Brevius Tractatus (St. Basil the Great)
Homily in Psalm 1 (St. Basil the Great)
Morals on the Book of Job, Book XI (St. Gregory the Great)