As described by Dietrich Bonhoeffer in The Cost of Discipleship, p. 211-212.
The path of discipleship is narrow, and it is fatally easy to miss one’s way and stray from the path, even after years of discipleship. And it is hard to find. On either side of the narrow path deep chasms yawn. To be called to a life of extraordinary quality, to live up to it, and yet be unconscious of it is indeed a narrow way. To confess and testify to the truth as it is in Jesus, and at the same time to love the enemies of that truth, his enemies and ours, and to love them with the infinite love of Jesus Christ, is indeed a narrow way. To believe the promise of Jesus that his followers shall possess the earth, and at the same time to face our enemies unarmed and defenseless, preferring to incur injustice rather than to do wrong ourselves, is indeed a narrow way. To see the weakness and wrong in others, and at the same time refrain from judging them; to deliver the gospel message without casting pearls before swine, is indeed a narrow way. The way is unutterably hard, and at every moment we are in danger of straying from it. If we regard this way as one we follow in obedience to an external command, if we are afraid of ourselves all the time, it is indeed an impossible way. But if we behold Jesus Christ going on before step by step, we shall not go astray. But if we worry about the dangers that beset us, if we gaze at the road instead of at him who goes before, we are already straying from the path. For he is himself the way, the narrow way and the straight gate. He, and he alone, is our journey’s end. When we know that, we are able to proceed along the narrow way through the straight gate of the cross, and on to eternal life, and the very narrowness of the road will increase our certainty. The way which the Son of God trod on earth, and the way which we too must tread as citizens of two worlds on the razor edge between this world and the kingdom of heaven, could hardly be a broad way. The narrow way is bound to be right.