Matthew 18:1-3

1At that time the disciples came to Jesus and asked, "Who is the greatest in the kingdom of heaven?" 2He called a little child and had him stand among them. 3And he said: "I tell you the truth, unless you change and become like little children, you will never enter the kingdom of heaven.

The disciples had been bickering about who was the greatest in heaven. Jesus overheard them (in Mark's gospel) and asked them what they were arguing about. Jesus spoke the truth to them. He told them three things:
1) you must change
2) become like children
3) if you don't follow steps 1 & 2, you will not enter the kingdom of heaven.

We all must change to be more child-like in our faith, in our openness to reach out to strangers and trust them, and our love. Children give the benefit of the doubt and accept each other. We are to have the faith of a child. In order to enter, we should change and become like a child.