To what shall I compare the kingdom of God?
It is like a farmer who promised his sons, when they were still children, to give each of them a car when they were old enough to drive. They didn't expect anything fancy, or expensive, or new - any old beater would do, so long as it was their own.
The boys were so excited by this prospect that they didn't mind having to do all the hard, hard work their father asked them to do. And when each of them turned 16, Dad made good his promise and either gave him one of his old cars or bought him an old beater.
Did these boys think they had earned their cars by doing all that work? They did not. They owed the cars to a promise their father had made prior to all their work. Yes, that promise sustained the sons through many an hour of hard work, but it was not conditioned on their work. Even after years of service, the sons had done no more than their duty. It was their father's generosity and good word they had to thank for the cars.
He who has ears, let him hear what the Spirit says in Romans, chapter 4. (Thanks again to WRS for suggesting this analogy.)