The disciples were having an argument. The question was not some deep theological issue. The question was "Who is greatest?" Every one of them thought they were greatest. None of them were. Finally, Jesus (who knew what they were thinking and what they were arguing about) stepped in.
It would have been easy to lambast and embarrass the disciples, but that's not what He did. (I should learn from Him on this) Jesus called a little boy to His side, and started teaching. The little boy was not rich or famous. He had no power or authority. He could not help advance their careers or their position. According to Thayer's Greek Lexicon, the word translated here as "receive" means "to take in to your home to raise and educate". If you receive (welcome and provide for) this child, you are doing it for Me. When you receive Me, you receive My Father who sent Me. The one who is least among you is the one who is great.
That word "least" can also mean little, as in little influence. The one who seems the least important, the most humble one among you, is the greatest. Both Princess Diana and Mother Teresa died about the same time. One woman was very wealthy and lived a very public life with a lavish lifestyle. One woman spent her life in poverty, caring for the least of the least. Both woman did many good things. Both woman made an impact on their worlds. Mother Teresa, however, spent her life loving the diseased and the dying, willing sacrificing herself to serve others. In the end, the least was the greatest.
Do you want to be great? If so, serve the least with great love. Greatness does not come with a big bank balance. It does not come with an important job or a fancy title. Greatness comes when we humble ourselves, roll up our sleeves, and care for the least likely with the greatest need. If you want to be great, you have to start by not only being least, but by loving and serving the least.