The narrow door
Though Jesus knew what lay before him at Jerusalem, he made his way there for his people (Luke 13:22). Butler says, "The trip to Jerusalem continues (Luke 9:51). The trip was used to meet Jesus' stated purpose: Let all the villages hear the word of the kingdom (Luke 4:43)." Of course, Jesus accomplished many missions on the way to the war at Calvary (Ezek. 34:12; Matt. 18:11; Luke 1:1; 19:10; John 3:16-18; 1 Tim. 1:15). Jesus had his face set to accomplish the Covenant of Redemption (see below). So, Luke gives a picture of Jesus making unhurried progress toward Jerusalem, where the climax of his life and mission would be realized. On the way Jesus continued to meet the needs of the people he encountered.
On the way someone asks, "Lord, are only a few people going to be saved?" (Luke 13:23). There were discussions among the Jews as to how many people would be saved (Luke 13:23), and it was generally agreed that all Israel (except for a few especially sinful people) would be included in that number. Jesus warned against such presumption.
Jesus replied, "Make every effort to enter through the narrow door, because many, I tell you, will try to enter and will not be able to. Once the owner of the house gets up and closes the door, you will stand outside knocking and pleading, 'Sir, open the door for us.' "But he will answer, 'I don't know you or where you come from.'" (Luke 13:24-25). Herein, Jesus says that "many" - not "some - of his hearers would not enter into the Kingdom of God.
Jews in Jesus' day felt privileged to be part of the covenant community. They had the Law and the prophets. As Paul says in Romans 9:3-5:
For I could wish that I myself were accursed and cut off from Christ for the sake of my brothers, my kinsmen according to the flesh. They are Israelites, and to them belong the adoption, the glory, the covenants, the giving of the law, the worship, and the promises. To them belong the patriarchs, and from their race, according to the flesh, is the Christ, who is God over all, blessed forever. Amen.
So, many Jews assumed salvation was a given. But Jesus demands personal faith, not mere heredity as the way to God. So, Jesus represents the way of salvation to the Jews as a narrow door (Luke 13:24; cf. Matt. 7:13-14).
If not for the Spirit of God, none would ever enter (John 3:3, 5) the Kingdom through God's narrow door. In salvation, though the Spirit of God alone, are the elect given spiritual eyes to see and spiritual legs to enter into the Kingdom of God.
Saved Noah, earnestly built an Ark (with one door, Gen. 6:16). Those reborn of the Spirit of God pursue faith diligently (Phil. 2:12; 2 Pet. 1:10). As Noah, those born of the Spirit persevere in faith as shown in Philippians 2:12-13 (see below). They work out their own salvation with fear and trembling (see below). One day the room will be shut and no one else will ever enter it (cf. compare Noah's Ark in Gen. 7:16). Some may knock, but once the door is shut, it remains shut and those outside suffer the risen tide of God's eternal judgment.
There is a very real danger in being a mere stranger. Jesus responds to such: "I don't know you or where you come from" (Luke 13:25) and "I don't know you or where you come from. Away from me, all you evildoers!" (Luke 13:27). Having social fellowship with Jesus and hearing his teaching were not enough. The people's attitude was superficial; they had never embraced the lordship of Christ. Mere outward contact with Jesus ('We ate and drank with you, and you taught in our streets') - mere knocking through church attendance and other works, etc. - means nothing; a personal relationship and inward reception is everything (Luke 6:46-49; John 1:12).
The judgment of the eternal Hell is coming (Luke 13:28) - see below. There will be weeping (in grief) and gnashing of teeth (in anger) as "many" see the great ones (with whom they had always classified themselves) in bliss, while they themselves are thrown out (which indicates God's active opposition to all that is evil).
In Luke 13:29 we observe that "people will come from east and west and north and south, and will take their places at the feast in the kingdom of God." These are believers, the saved, the elect, those shut in the eternal Ark - secure and safe. The saved will include people from all over the world, including Gentiles - "persons from every tribe and language and people and nation" (Rev. 5:9; 7:9). We see a feast, the imagery of the Messianic banquet that will take place when Christ returns. This image reflects the great joy that will be present in the Kingdom at that time (Luke 14:15; Rev. 19:9) - as compared to this "outside" (Luke 13:28). There will be a complete reversal of many ideas strongly held among humankind.