What does Jesus mean by saying, “The last shall be first and the first shall be last ?”
Children of Promise
During Jesus’ earthly ministry, the children of promise, or the children of Abraham, already considered themselves part of the coming kingdom. The Jews thought it was by Abraham that they would be saved, being the “children of promise” (Rom 9:8), and the Apostle Paul even acknowledges them as “brothers, like Isaac, are children of promise” (Gal 4:28), but Paul is not saying that they are saved because of their ancestry. No one gets into the kingdom by someone else’s coattails. There are only saints and “aint’s.” They have either trusted in Christ or they have trusted in Abraham, their patriarchal father, but it is not by race we are saved; it is by grace we are saved (Eph 2:8-9). Paul quotes the Prophet Hosea who wrote, “Those who were not my people I will call ‘my people,’ and her who was not beloved I will call ‘beloved” (Rom 9:28). Clearly, “those who were not my people” includes Jew and Gentile, and not just the descendants of Abraham. Many of Abraham’s descendants were saved in the first century, but it wasn’t by Abraham. They were saved by repentance and faith. Sadly, the religious leaders and many of the Jews thought they were going to be first in the kingdom since they were Abraham’s descendants, but Jesus will reveal that many who thought they were first will end up being last, and the last will be first. 
The Servant’s the Greatest
This is one teaching the Jewish religious leaders never learned. They failed to see how they should be serving others. They focused on others serving them; others elevating them; others giving them the seats of honor. This was just the opposite of what Jesus taught. When Paul and Silas came in preaching the gospel, the authorities said, “These men who have turned the world upside down have come here also” (Acts 17:6), however if you think about it, it’s actually turning the world right side up! Part of being in the leadership includes serving those around you, not making sure they serve you. In God’s eyes, the greatest aren’t those with the largest number of servants, but those who have served the largest number. Naturally, Jesus’ teaching is contrary to the ways of the world, and that’s because Jesus is not part of the world. His ways are not the ways of man (Isaiah 55:8-9; Eph 3:20-21), and neither were they with the Jews of Jesus’ day.
Who Are the First?
Jesus warned the religious leaders (and all others) that “many who are first will be last, and the last first” (Matt 20:16), so who are the first and who are the last? Jesus was just concluding the Parable of the Vineyard, and the workers were obviously the Jews who were hired first to keep the vineyard and have long been associated as the chosen children of God, or the children of promise. They have endured long suffering over the last few thousand years, but that doesn’t give them an automatic entrance into heaven, nor does it merit them worthy on the Day of Judgment when Christ shall reward His church. When the disciples were arguing about who would be the greatest in the kingdom, Jesus said that “everyone who has left houses or brothers or sisters or father or mother or children or lands, for my name’s sake, will receive a hundredfold and will inherit eternal life” (Matt 19:29), however it’s not the way they expect because our Lord said, “many who are first will be last, and the last first” (Matt 19:30).
The Last and First
Those who believed they are going to be first or one of the foremost in the kingdom, like the Jews thought, may actually end up being last. The last, later Gentile converts and those saved up to today, will be first in the kingdom in comparison to those who think of themselves first. Quite a shock is in store for the Jews of Jesus’ day, so a quick reminder from Jesus tells us that “Whoever exalts himself will be humbled, and whoever humbles himself will be exalted” (Matt 23:12). It seems better to humble ourselves today than to have God do it for us tomorrow. The proud Jewish religious leaders will be humbled on the Day they see Jesus. The exalted ones shall be abased, while the abased (that’s us), shall be exalted, but it will be by God Who exalts, not we ourselves. God is opposed to every proud heart (James 4:6).
A braham’s bosom has long been a Jewish symbol of Paradise or as close to heaven as you can get, but Jesus once again angers the religious leaders, saying that “many will come from east and west and recline at table with Abraham, Isaac, and Jacob in the kingdom of heaven” (Matt 8:11), but what about the Jew’s? Jesus said the many which came from the east and west, or those who are Gentiles, will dine in the kingdom of heaven with the patriarchs, “while the sons of the kingdom will be thrown into the outer darkness. In that place there will be weeping and gnashing of teeth” (Matt 8:12). Obviously those from the east and west included the Roman Centurion who Jesus had just been speaking with, and whose servant He had healed, so “God’s people” (children of Abraham) would be last, while the Gentiles would be first in the kingdom. Isn’t that the way of God; always contrary to the ways of the world!
If our parents or grandparents, or brothers and sisters, or children are believers, but we’re not, we have no entitlement to enter the kingdom. The Jews couldn’t enter just because they were Abraham’s children and neither can we, even if everyone in our family is saved. As for striving to be first in the kingdom as the religious leaders did, the greatest will be those who have served and sacrificed the most, not those who have the most serving them. That’s servant leadership. You cannot be a leader and not be called to serve, and we are all called to serve those in need (Matt 25:35-36), doing it as unto Jesus (Matt 25:40). We are to go to the least of those in the world, and the least of these our brothers and sisters, but Jesus warned the self-righteous Jews that “the tax collectors and the prostitutes go into the kingdom of God before you” (Matt 21:31), so it’s not by race but only by grace that we are saved (Eph 2:8-9). That’s the only way that the last shall be first.
Here is some related reading for you: What is Servant Leadership: A Christian Study 
Resource – Scripture quotations are from The Holy Bible, English Standard Version® (ESV®), Crossway Bibles. (2007). ESV: Study Bible: English standard version. Wheaton, Ill: Crossway Bibles. Used by permission. All rights reserved.