The earlier portion of Luke 13, vss 18-20, seem to be pointing to the kingdom of God expanding throughout the world as the gospel is spread. The mustard seed becoming a tree and the leaven leavening all the flour are used as an example of the kingdom of God expanding as the gospel is spread far and wide.
Vs 23 and onward I am not for sure if this is also referring to the kingdom of God here or in the eternal state. Vs 24 Jesus said "Strive to enter through the narrow door; for many, I tell you, will seek to enter and will not be able." I do not believe this to be the eternal state as none will be able to strive to enter it after death and the day of judgment. So, this seems to me to be pointing to the kingdom of God here on earth. That where He said "many, I tell you. will seek to enter and will not be able", I tend to believe this is referring to ppl who seek other ways to God, and do not choose to go through the Door of the sheep pen, Christ. But vs 25 onward I still need to study as I do not have an adequate answer to give you at this time.
This is the complete passage in Luke:
And one said to Him, Lord, are the ones being saved few? And He said to them,
24 Strive to enter in at the narrow gate. For I say to you, many will seek to enter in and shall not be able.
25 And once the Master of the house has risen up and has shut the door, and you begin to stand outside and to knock at the door, saying, Lord, Lord, open to us, and He shall answer and say to you, I do not know you; from where are you;
26 then you shall begin to say, We ate and drank in Your presence, and You have taught in our streets.
27 But He shall say, I tell you, I do not know you; from where you are. Depart from Me, all workers of unrighteousness!
28 There will be weeping and gnashing of teeth when you will see Abraham and Isaac and Jacob, and all the prophets, in the kingdom of God, and yourselves being thrust out.
29 And they will come from the east and the west, and from the north and the south, and will recline in the kingdom of God.
30 And behold, there are last ones which will be first, and there are first ones which will be last.
Clearly it is talking about "The Kingdom of God", and the After life.
This is an illustration as a warning to those who are professing Christians, who think that they are saved, because of their good works, when in reality, they are not.
Jesus is using metaphors, like the narrow gate, striving to enter, knocking outside the door, seeing the Prophets in the Kingdom of God, etc. to describe the reality of salvation and eternal life.
Because literally there can be no person who can try to enter heaven after they have died, which is clear from Hebrews 9:27. Nor will anyone literally be trying to argue with the Lord after they have died, as to why they did not make it to heaven, as they thought they were saved, by the many good works they had done in His Name.
Like in Matthew 22:11-13,
11 “But when the king came in to see the guests, he noticed a man there who was not wearing wedding clothes. 12 He asked, ‘How did you get in here without wedding clothes, friend?’ The man was speechless.
13 “Then the king told the attendants, ‘Tie him hand and foot, and throw him outside, into the darkness, where there will be weeping and gnashing of teeth.’
It is not possible to have entered the Kingdom of God, without being saved, which is account in Matthew is showing. This is an illustration of those who suppose there is another Way to get to heaven. As in John chapter 10, where Jesus is The Door, and says that no one can get into heaven by any other means, but through Him.
