It is not uncommon for Jesus to summon people to “enter” the kingdom of God. Note Mark 9:47; 10:15; 14:25 as well as Matthew5:19; 7:21; 8:11; 18:3. The most significant text of all these, and others could be listed, is Matthew 7:21: “Not everyone who says to me, ‘Lord, Lord,’ will enter the kingdom of heaven, but only the one who does the will of my Father who is in heaven.” Within the scope of the concluding summons to the Sermon, “enter” (the Greek word is eiserchomai) is used for both “life” and “kingdom.” The gravity of the summons is palpable: Jesus calls his followers to enter into the life of the kingdom in the here and now.
Everything hinges on the “gate,” the entrance to cities and temples. What does this mean? Some restrict “gate” to the ethical vision or commands of Jesus from 5:1 to 7:12, while others see Jesus himself along with his commands. Jesus is the gate in John 10:9, and he is calling people to follow him; his demands are entailed in relationship with Jesus. The Messianic Ethic creates the Ethic from Above.
There is one reason the gate is “narrow”: it is demanding discipleship. It is demanding because it contrasts with the wide gate that leads to a broad road in 7:13b and compares favorably with the “small” gate that leads to a narrow road in 7:14. The gate is narrow because it requires a person to turn from sin to follow Jesus, to do the will of God as taught by Jesus. It is narrow because it is the surpassing righteousness of 5:17-48, the deeper righteousness of 6:1-18, the single-minded righteousness of 6:19-34, and the wise way of life as seen in 7:1-11. In essence, the narrow gate is to follow Jesus by learning to live by the Jesus Creed and the Golden Rule (7:12). And the gate is narrow because of persecutions (5:11-12; 8:18-22; 10:17-25, 34-39; 24:9, 21, 29). Is it possible to walk the narrow road?
“As long as I recognize this road as the one I am commanded to walk, and try to walk it in fear of myself, it is truly impossible. But if I see Jesus Christ walking ahead of me, step by step, if I look only at him and follow him, step by step, then I will be protected on this path.”
Scot McKnight, Sermon on the Mount (The Story of God Bible Commentary). The last quote: Dietrich Bonhoeffer, Discipleship, 176.