"Truly, I say to you, whoever does not receive the kingdom of God like a child shall not enter it.”
This is one of the more enigmatic statements of Jesus and one of 11 Amen sayings found in Mark. With a narrow minded focus, it is not apparent what Jesus means by "like a child", but a closer look reveals that Mark illustrates exactly what Jesus means. It is no accident that the parable of the Rich young ruler follows this endearing story of Jesus and the little children. The contrast is clear between the children and the rich young ruler, those who have nothing and those who have everything.
We see this rich man failing to enter into the kingdom of God despite receiving a personal invitation from Christ because he could not receive it like a child. Jesus makes a synonymous statement in Mark 10:23 "How hard it is for the rich to enter the kingdom of God!"
The kingdom belongs to those who would give up everything to follow Christ (for some it is more than others). It belongs to those who are able to follow and submit like little children in obedience to their Father. Humility is the trait that allows one to submit to a greater authority and be taught by One wiser than yourself. To be humble is to simply know where you stand in relation to God. One gets to the kingdom by following the One who knows the way, Jesus Christ.