Have you ever heard someone say, "Stop acting like a child!" Or, have you ever told someone, "Act your age?" In the eyes of the world, adults should act like adults, not like kids. When we become adults, we need to step up and shoulder the responsibilities that come with adulthood. We are expected to display the maturity of an adult rather than the immaturity of a child. All of these things are good and true. But, when it comes to the kingdom of heaven, it is better for us to be like a child.
When His disciples asked Him who was greatest in the kingdom of heaven, Jesus called a child to Him. Putting the child in the midst of the disciples, Jesus said, "Truly, I tell you, unless you turn and become like children, you will never enter the kingdom of heaven. Whoever humbles himself like this child is the greatest in the kingdom of heaven." (Matthew 18:3-4, ESV). Jesus was telling His disciples, and us, that we need to change the way we think.
We need to turn away from the idea that the things that we do, or the things that we say, can make us great in God's kingdom. Instead, we need to display the humility of a child. Children are reliant on others, dependent on their parents or other adults for the things that they need. Basically, they are at the mercy of adults.
When we become dependent on God, when we place ourselves at His mercy and admit that, without Him, we can do nothing, we make ourselves as humble as a child. And that is what Jesus is looking for in us. Humble hearts, like those of children. In fact, He tells us that, if we do not receive the kingdom of God with the humility of a child, we will not enter His kingdom (Luke 18:17).