"We ate and drank in Your presence, and You taught in our streets" (13:26). These words would be spoken in response to the Master's claim of not knowing them. Although their response is true, they do not genuinely know God because they refused to heed His words! Yes, the Jews had seen and heard Jesus teaching publicly. Yes, they had shared meals with Him. But, no, they were not "known" by the Lord because they refused to obey God. They had rejected Him and judged themselves unworthy of His blessings--including eternal life (e.g., Acts 13:46).
Although they had been given many privileges to obey God and be pleasing to Him, they had failed to make proper use of them. This is why God does not "know" them. The "door" of salvation remained shut, and they, as workers of sin are paid their "wages" in being told to depart (cf. Rom. 6:23). "Depart from Me, all you workers of iniquity" (Luke 13:27). Let it be understood by all that at the judgment no amount of pleading will change God's mind regarding one's eternal destiny. The door is rightly called "narrow," and it would not be such if it opened up to excuses.
"There will be weeping and gnashing of teeth, when you see Abraham and Isaac and Jacob and all the prophets in the kingdom of God, and yourselves thrust out" (Luke 13:28; cf. Matt. 8:12). The "weeping and gnashing of teeth" describes the intense suffering that those who do not know God will endure (cf. II Thess. 1:7-9). They will weep because they now realize, after it is too late, that they are lost. They will gnash their teeth because they will "see" the faithful (e.g., Abraham, Isaac, Jacob, the prophets, etc.) in the kingdom of God, but not themselves. The agony they will experience spiritually cannot rightly be compared to any earthly suffering. The pain will only be increased for some when they reflect upon the fact that they could have been a part of the kingdom but rejected the Lord and were thus "thrust out." Others will mourn the fact that they had intended to make proper spiritual preparations but, due to procrastination, the "door" closed sooner than they anticipated.
Luke 13:29 reads - "They will come from the east and the west, from the north and the south, and sit down in the kingdom of God." In this context, "they" refers to the Gentiles and "the kingdom of God" refers to heaven.
"There are last who will be first, and there are first who will be last" (Luke 13:30; cf. Matthew 19:30; 21:31; Rom. 9:30,31). Jews who thought that the Gentiles had no hope at all of salvation will be surprised when they find out that just the opposite is true. Those who are "first" here on earth are those who are viewed as important and prestigious. Those who are "last" here on earth are those who are viewed as insignificant. However, those who are viewed as "first" here will often end up "last" in eternity (i.e., in eternal condemnation), and those who are viewed as "last" here will often end up "first" in eternity (i.e., in eternal glory).