Matthew 18:1-5

Mat 18:1 (KJB)
At the same time came the disciples unto Jesus, saying, Who is the greatest in the kingdom of heaven?

It seems from the type of question which was being asked, the disciples were still looking for an earthly kingdom and that may have prompted them to ask the question about the greatest one in the kingdom of Heaven. Previous to this Peter was called a rock and then he, John and James were taken up to the Mount of Transfiguration and this may have added to the desire to know who is the greatest.

Mat 18:2 (KJB)
And Jesus called a little child unto him, and set him in the midst of them,

Then Jesus gives them an object lesson by setting a child in the midst of them. There is an old unreliable tradition that this child was Ignatius of Antioch. (Mark 9:36 KJV) And he took a child, and set him in the midst of them: and when he had taken him in his arms, he said unto them, Jesus had embraced the little child according to Mark’s account.

Mat 18:3 (KJB)
And said, Verily I say unto you, Except ye be converted, and become as little children, ye shall not enter into the kingdom of heaven.

The first requirement for even being in the Kingdom of Heaven is that one must be converted, that is, one must became saved through the Lord Jesus Christ. Then secondly one of the characteristics of being a true child of God is that one must become as a little child. They were to become like children of God who are always dependent upon the heavenly Father. The object lesson here is that one be like a child. This verse is not teaching that all children are automatically children of God, they too, must become saved in the same way an adult becomes saved.

Mat 18:4 (KJB)
Whosoever therefore shall humble himself as this little child, the same is greatest in the kingdom of heaven.

Then once a person is in the kingdom of God, they are to maintain a humble attitude. A child is trusting, dependent, looks to his parents, respects his parents. When a person becomes lowly in heart and spirit in the Kingdom of God, then they are considered the greatest. God can accomplish much through a person who is truly humble but He can accomplish little through those who are proud or arrogant. To become greatest in the kingdom of Heaven, one must be least of all.

Mat 18:5 (KJB)
And whoso shall receive one such little child in my name receiveth me.

In many cultures today and in ancient times, children were viewed upon as insignificant and unimportant. So many times children were rejected. Jesus is stating that if anyone receives one of His children, one who is born again, receives Him. Not necessarily in salvation but as one who is not hostile to the Gospel in hopes it leads him to salvation. The world sees true Christians as insignificant and unimportant so when one is accepted or welcomed, they are welcoming Christ too. Just as Jesus told Paul on the road to Damascus that by persecuting the Christians, Paul was persecuting Him. The other side of the coin is that those who welcome Christians are welcoming Christ. We saw this with Joseph because Potiphar’s house was blessed because of Joseph. (Gen 39:5 KJV) And it came to pass from the time that he had made him overseer in his house, and over all that he had, that the LORD blessed the Egyptian's house for Joseph's sake; and the blessing of the LORD was upon all that he had in the house, and in the field.